View source for Philosophy:Ontology

{{Short description|Philosophical study of being and existence}}
[[File:Cartesian_Theater.svg|right|thumb|upright=1.35|An illustration of [[Philosophy:Cartesian materialism|Cartesian materialism]], which argues that it is possible to find the content of conscious experience moment by moment in the mind. Materialism in general, arguing that matter is the fundamental 'substance', is an influential perspective on ontology.]]
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In [[Philosophy:Metaphysics|metaphysics]], '''ontology''' is the [[Philosophy:Philosophy|philosophical]] study of being. It investigates what types of entities exist, how they are grouped into [[Philosophy:Category of being|categories]], and how they are related to one another on the most fundamental level (and whether there even is a fundamental level).<ref>{{Cite journal |last=Schaffer |first=Jonathan |date=2003-07-18 |title=Is There a Fundamental Level? |url=http://dx.doi.org/10.1111/1468-0068.00448 |journal=Noûs |volume=37 |issue=3 |pages=498–517 |doi=10.1111/1468-0068.00448 |issn=0029-4624}}</ref> Ontologists often try to determine what the categories or highest kinds are and how they form a system of categories that encompasses the classification of all entities. Commonly proposed categories include substances, [[Philosophy:Property|properties]], [[Philosophy:Relations|relations]], [[Philosophy:State of affairs|states of affairs]], and [[Philosophy:Event|events]]. These categories are characterized by fundamental ontological concepts, including particularity and universality, abstractness and concreteness, or possibility and necessity. Of special interest is the concept of ontological dependence, which determines whether the entities of a category exist on the most fundamental level. Disagreements within ontology are often about whether entities belonging to a certain category exist and, if so, how they are related to other entities.<ref name="Hofweber" />

When used as a countable noun, the words ''ontology'' and ''ontologies'' refer not to the science of being but to theories within the science of being. Ontological theories can be divided into various types according to their theoretical commitments. Monocategorical ontologies hold that there is only one basic category, but polycategorical ontologies rejected this view. Hierarchical ontologies assert that some entities exist on a more fundamental level and that other entities depend on them. Flat ontologies, on the other hand, deny such a privileged status to any entity.
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== Etymology ==

The compound word ''ontology'' ('study of being') combines
:''onto-'' ([[Social:Greek language|Greek]]: {{Lang-gr|ὄν|label=none|translit=on}};<!--[sic] (neuter), not ὤν (masculine)-->{{NoteTag|ὄν is the present-tense participle of the verb εἰμί (''eimí'', 'to be' or 'I am').}} {{Abbr|{{sc|GEN}}.|Genitive case}} {{Lang-gr|ὄντος|label=none|translit=ontos|lit=being' or 'that which is}}) and
:''-logia'' ({{Lang-gr|{{wikt-lang|en|-λογία}}|lit=logical discourse|label=none}}).<ref name="OnlineEtDict">{{cite dictionary |title = ontology |url = https://www.etymonline.com/word/ontology |dictionary = Online Etymology Dictionary}}</ref><ref name="LSJ">{{LSJ|ei)mi/1|εἰμί|ref}}.</ref>

While the [[Social:Etymology|etymology]] is Greek, the oldest extant records of the word itself is a Neo-Latin form {{Lang-la|ontologia|label=none}}, which appeared
:in 1606 in the [https://www.google.it/books/edition/Ogdoas_Scholastica_continens_diagraphen/EIvc1kak6xEC ''Ogdoas Scholastica''] by [[Biography:Jacob Lorhard|Jacob Lorhard]] (''Lorhardus''), and
:in 1613 in the ''Lexicon philosophicum'' by [[Biography:Rudolph Goclenius|Rudolf Göckel]] (''Goclenius'').
The first occurrence in English of ''ontology'', as recorded by the ''Oxford English Dictionary'',<ref>{{cite web |url=https://www.oxfordlearnersdictionaries.com/us/definition/english/ontology?q=ontology |title=ontology|website=Oxford Advanced Learner's Dictionary |publisher=[[Organization:Oxford University Press|Oxford University Press]] |year=2008}}</ref> came in 1664 through ''Archelogia philosophica nova...'' by Gideon Harvey.<ref>{{Cite web |title=Archelogia philosophica nova, or, New principles of philosophy containing philosophy in general, metaphysicks or ontology, dynamilogy or a discourse of power, religio philosophi or natural theology, physicks or natural philosophy / by Gideon Harvey ... |url=https://quod.lib.umich.edu/cgi/t/text/text-idx?c=eebo;idno=A43008.0001.001 |access-date=2023-09-30 |website=quod.lib.umich.edu}}</ref> The word was first used, in its Latin form, by philosophers, and based on the Latin roots (and in turn on the Greek ones).

== Overview<!--'Eleatic principle' redirects here--> ==
Ontology is closely associated with Aristotle's question of 'being ''qua'' being': the question of what all entities in the widest sense have in common.<ref name="Borchert2"/><ref>{{cite book |last1=Sandkühler |first1=Hans Jörg |url=https://meiner.de/enzyklopadie-philosophie.html |title=Enzyklopädie Philosophie |date=2010 |publisher=Meiner |language=de |chapter=Ontologie: 2.1 Antike |quote=Nach einer berühmten Formulierung von Aristoteles (384–322 v. Chr.), der zwar wie auch Platon nicht den Ausdruck ›O.‹ verwendet, sich jedoch der Sache nach in seiner ›ersten Philosophie‹ ausführlich damit befasst, lässt sich O. charakterisieren als die Untersuchung des Seienden als Seiendem (to on he on). |access-date=2020-12-16 |archive-url=https://web.archive.org/web/20210311040207/https://meiner.de/enzyklopadie-philosophie.html |archive-date=2021-03-11 |url-status=dead}}</ref> The '''Eleatic principle'''<!--boldface per WP:R#PLA--> is one answer to this question: it states that being is inextricably tied to causation, that "Power is the mark of Being".<ref name="Borchert2">{{cite book |last1=Borchert |first1=Donald |title=Macmillan Encyclopedia of Philosophy, 2nd Edition |date=2006 |publisher=Macmillan |url=https://philpapers.org/rec/MONMEO-3 |chapter=Ontology}}</ref> One problem with this answer is that it excludes abstract objects. Another explicit but little-accepted answer can be found in Berkeley's slogan that "to be is to be perceived".<ref>{{cite web |last1=Flage |first1=Daniel E. |title=Berkeley, George |url=https://iep.utm.edu/berkeley/ |website=Internet Encyclopedia of Philosophy |access-date=4 January 2021}}</ref> Intimately related but not identical to the question of 'being ''qua'' being' is the problem of [[Philosophy:Category of being|categories]].<ref name="Borchert2"/> Categories are usually seen as the highest kinds or genera.<ref name="Thomasson2">{{cite web |last1=Thomasson |first1=Amie |title=Categories |url=https://plato.stanford.edu/entries/categories/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=4 January 2021 |date=2019}}</ref> A system of categories provides a classification of entities that is exclusive and exhaustive: every entity belongs to exactly one category. Various such classifications have been proposed, they often include categories for [[Philosophy:Substance theory|substances]], [[Philosophy:Property|properties]], [[Philosophy:Relations|relations]], [[Philosophy:State of affairs|states of affairs]] or [[Philosophy:Event|events]].<ref name="Borchert2"/><ref name="Sandkühler2">{{cite book |last1=Sandkühler |first1=Hans Jörg |url=https://meiner.de/enzyklopadie-philosophie.html |title=Enzyklopädie Philosophie |date=2010 |publisher=Meiner |language=de |chapter=Ontologie: 4 Aktuelle Debatten und Gesamtentwürfe |access-date=2020-12-16 |archive-url=https://web.archive.org/web/20210311040207/https://meiner.de/enzyklopadie-philosophie.html |archive-date=2021-03-11 |url-status=dead}}</ref> At the core of the differentiation between categories are various fundamental ontological concepts and distinctions, for example, the concepts of ''particularity and universality'', of ''abstractness and concreteness'', of ''ontological dependence'', of ''identity'' and of ''modality''.<ref name="Borchert2"/><ref name="Sandkühler2"/> These concepts are sometimes treated as categories themselves, are used to explain the difference between categories or play other central roles for characterizing different ontological theories. Within ontology, there is a lack of general consensus concerning how the different categories are to be defined.<ref name="Thomasson2"/> Different ontologists often disagree on whether a certain category has any members at all or whether a given category is fundamental.<ref name="Sandkühler2"/>

=== Particulars and universals ===
[[Physics:Particular|Particular]]s or individuals are usually contrasted with [[Philosophy:Universal (metaphysics)|universals]].<ref>{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=particulars and non-particulars}}</ref><ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Particulars}}</ref> Universals concern features that can be exemplified by various different particulars.<ref name="MacLeod">{{cite web |last1=MacLeod |first1=Mary C. |title=Universals |url=https://iep.utm.edu/universa/ |website=Internet Encyclopedia of Philosophy |access-date=4 January 2021}}</ref> For example, a tomato and a strawberry are two particulars that exemplify the universal redness. Universals can be present at various distinct locations in space at the same time while particulars are restricted to one location at a time. Furthermore, universals can be fully present at different times, which is why they are sometimes referred to as ''repeatables'' in contrast to non-repeatable particulars.<ref name="Sandkühler2"/> The so-called [[Philosophy:Problem of universals|problem of universals]] is the problem to explain how different things can agree in their features, e.g. how a tomato and a strawberry can both be red.<ref name="Borchert2"/><ref name="MacLeod"/> [[Philosophy:Problem of universals#realism|Realists]] believe that there are universals. They can solve the ''problem of universals'' by explaining the commonality through a universal shared by both entities.<ref name="Sandkühler2"/> Realists are divided among themselves as to whether universals can exist independently of being exemplified by something ("ante res") or not ("in rebus").<ref>{{cite web |title=Realism – Universals |url=https://www.britannica.com/topic/realism-philosophy/Universals |website=Encyclopedia Britannica |access-date=4 January 2021 |language=en}}</ref> [[Philosophy:Nominalism|Nominalists]], on the other hand, deny that there are universals. They use other notions to explain how a feature can be common to several entities, for example, by positing either fundamental resemblance-relations between the entities (resemblance nominalism) or a shared membership to a common natural class (class nominalism).<ref name="Sandkühler2"/>

=== Abstract and concrete ===
{{main|Philosophy:Abstract and concrete}}
Many philosophers agree that there is an exclusive and exhaustive distinction between ''concrete objects'' and ''abstract objects''.<ref name="Sandkühler2"/> Some philosophers consider this to be the most general division of being.<ref name="Honderich">{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=Ontology}}</ref> Examples of concrete objects include plants, human beings and planets while things like numbers, sets and propositions are abstract objects.<ref>{{cite web |last1=Rosen |first1=Gideon |title=Abstract Objects |url=https://plato.stanford.edu/entries/abstract-objects/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=4 January 2021 |date=2020}}</ref> But despite the general agreement concerning the paradigm cases, there is less consensus as to what the characteristic marks of concreteness and abstractness are. Popular suggestions include defining the distinction in terms of the difference between (1) existence inside or outside [[Physics:Spacetime|spacetime]], (2) having causes and effects or not and (3) having contingent or necessary existence.<ref>{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=abstract entities}}</ref><ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Abstract objects}}</ref>

=== Ontological dependence ===

{{main|Philosophy:Grounding (metaphysics)}}

An entity ''ontologically depends'' on another entity if the first entity cannot exist without the second entity. Ontologically independent entities, on the other hand, can exist all by themselves.<ref>{{cite book |last1=Sandkühler |first1=Hans Jörg |url=https://meiner.de/enzyklopadie-philosophie.html |title=Enzyklopädie Philosophie |date=2010 |publisher=Meiner |language=de |section=Ontologie: 4.2.3 Ontologische Unabhängigkeit. Ganz grob gesagt versteht man unter existenzieller oder ontologischer (im Gegensatz z.B. zu logischer) Unabhängigkeit die Fähigkeit, ›alleine zu existieren‹. |access-date=2020-12-16 |archive-url=https://web.archive.org/web/20210311040207/https://meiner.de/enzyklopadie-philosophie.html |archive-date=2021-03-11 |url-status=dead}}</ref> For example, the surface of an apple cannot exist without the apple and so depends on it ontologically.<ref>{{cite web |last1=Varzi |first1=Achille |title=Boundary |url=https://plato.stanford.edu/entries/boundary/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=4 January 2021 |date=2015}}</ref> Entities often characterized as ontologically dependent include properties, which depend on their bearers, and boundaries, which depend on the entity they demarcate from its surroundings.<ref name="Tahko">{{cite web |last1=Tahko |first1=Tuomas E. |last2=Lowe |first2=E. Jonathan |title=Ontological Dependence |url=https://plato.stanford.edu/entries/dependence-ontological/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=4 January 2021 |date=2020}}</ref> As these examples suggest, ontological dependence is to be distinguished from causal dependence, in which an effect depends for its existence on a cause. It is often important to draw a distinction between two types of ontological dependence: rigid and generic.<ref name="Tahko"/><ref name="Sandkühler2"/> Rigid dependence concerns the dependence on one specific entity, as the surface of an apple depends on its specific apple.<ref name="Erices">{{cite journal |last1=Erices |first1=Gonzalo Nuñez |title=Boundaries and Things. A Metaphysical Study of the Brentano-Chisholm Theory |journal=Kriterion: Journal of Philosophy |date=2019 |volume=33 |issue=2 |pages=15–48 |doi=10.1515/krt-2019-330203 |s2cid=245494576 |url=https://philpapers.org/rec/ERIBAT|doi-access=free }}</ref> Generic dependence, on the other hand, involves a weaker form of dependence, on merely a certain type of entity. For example, electricity generically depends on there being charged particles, but it does not depend on any specific charged particle.<ref name="Tahko"/> Dependence-relations are relevant to ontology since it is often held that ontologically dependent entities have a less robust form of being. This way a ''hierarchy'' is introduced into the world that brings with it the distinction between more and less fundamental entities.<ref name="Tahko"/>

=== Identity ===
[[Philosophy:Identity|Identity]] is a basic ontological concept that is often expressed by the word "same".<ref name="Sandkühler2"/><ref name="Noonan">{{cite web |last1=Noonan |first1=Harold |last2=Curtis |first2=Ben |title=Identity |url=https://plato.stanford.edu/entries/identity/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=4 January 2021 |date=2018}}</ref> It is important to distinguish between ''qualitative identity'' and ''numerical identity''. For example, consider two children with identical bicycles engaged in a race while their mother is watching. The two children have the ''same'' bicycle in one sense (''qualitative identity'') and the ''same'' mother in another sense (''numerical identity'').<ref name="Sandkühler2"/> Two qualitatively identical things are often said to be indiscernible. The two senses of ''sameness'' are linked by two principles: the principle of ''indiscernibility of identicals'' and the principle of ''[[Philosophy:Identity of indiscernibles|identity of indiscernibles]]''. The principle of ''indiscernibility of identicals'' is uncontroversial and states that if two entities are numerically identical with each other then they exactly resemble each other.<ref name="Noonan"/> The principle of ''identity of indiscernibles'', on the other hand, is more controversial in making the converse claim that if two entities exactly resemble each other then they must be numerically identical.<ref name="Noonan"/> This entails that "no two distinct things exactly resemble each other".<ref>{{cite web |last1=Forrest |first1=Peter |title=The Identity of Indiscernibles |url=https://plato.stanford.edu/entries/identity-indiscernible/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=20 January 2021 |date=2020}}</ref> A well-known counterexample comes from [[Biography:Max Black|Max Black]], who describes a symmetrical universe consisting of only two spheres with the same features.<ref>{{cite journal |last1=Black |first1=Max |title=The Identity of Indiscernibles |journal=Mind |date=1952 |volume=61 |issue=242 |pages=153–164 |doi=10.1093/mind/LXI.242.153 |url=https://philpapers.org/rec/BLATIO-5}}</ref> Black argues that the two spheres are indiscernible but not identical, thereby constituting a violation of the principle of ''identity of indiscernibles''.<ref>{{cite journal |last1=Cowling |first1=Sam |title=Non-Qualitative Properties |journal=Erkenntnis |date=2015 |volume=80 |issue=2 |pages=275–301 |doi=10.1007/s10670-014-9626-9 |s2cid=122265064 |url=https://philpapers.org/rec/COWNP}}</ref>

The problem of ''identity over time'' concerns the question of ''persistence'': whether or in what sense two objects at different times can be ''numerically identical''. This is usually referred to as ''diachronic identity'' in contrast to ''synchronic identity''.<ref name="Noonan"/><ref name="Gallois">{{cite web |last1=Gallois |first1=Andre |title=Identity Over Time |url=https://plato.stanford.edu/entries/identity-time/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=23 January 2021 |date=2016}}</ref> The statement that "[t]he table in the next room is identical with the one you purchased last year" asserts diachronic identity between the table now and the table then.<ref name="Gallois"/> A famous example of a denial of diachronic identity comes from [[Biography:Heraclitus|Heraclitus]], who argues that it is impossible to step into the same river twice because of the changes that occurred in-between.<ref name="Noonan"/><ref name="Costa"/> The traditional position on the problem of ''persistence'' is [[Philosophy:Endurantism|endurantism]], the thesis that diachronic identity in a strict sense is possible. One problem with this position is that it seems to violate the principle of ''indiscernibility of identicals'': the object may have undergone changes in the meantime resulting in it being discernible from itself.<ref name="Sandkühler2"/> [[Philosophy:Perdurantism|Perdurantism]] or [[Four-dimensionalism|four-dimensionalism]] is an alternative approach holding that ''diachronic identity'' is possible only in a loose sense: while the two objects differ from each other strictly speaking, they are both temporal parts that belong to the same temporally extended whole.<ref name="Sandkühler2"/><ref>{{cite web |last1=Hawley |first1=Katherine |title=Temporal Parts |url=https://plato.stanford.edu/entries/temporal-parts/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=23 January 2021 |date=2020}}</ref> ''Perdurantism'' avoids many philosophical problems plaguing ''endurantism'', but ''endurantism'' seems to be more in touch with how we ordinarily conceive ''diachronic identity''.<ref name="Gallois"/><ref name="Costa">{{cite web |last1=Costa |first1=Damiano |title=Persistence in Time |url=https://iep.utm.edu/per-time/ |website=Internet Encyclopedia of Philosophy |access-date=23 January 2021}}</ref>

=== Modality ===
[[Modal logic|Modality]] concerns the concepts of possibility, actuality and necessity. In contemporary discourse, these concepts are often defined in terms of possible worlds.<ref name="Sandkühler2"/> A possible world is a complete way how things could have been.<ref>{{cite journal |last1=Vander Laan |first1=David A. |date=1997 |title=The Ontology of Impossible Worlds |url=https://philpapers.org/rec/VANTOO |journal=Notre Dame Journal of Formal Logic |volume=38 |issue=4 |pages=597–620 |doi=10.1305/ndjfl/1039540772 |doi-access=free}}</ref> The actual world is one possible world among others: things could have been different from what they actually are. A proposition is possibly true if there is at least one possible world in which it is true; it is necessarily true if it is true in all possible worlds.<ref>{{cite web |last1=Menzel |first1=Christopher |title=Possible Worlds |url=https://plato.stanford.edu/entries/possible-worlds/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=5 January 2021 |date=2017}}</ref> ''[[Philosophy:Actualism|Actualists]]'' and ''possibilists'' disagree on the ''ontological status'' of possible worlds.<ref name="Sandkühler2"/> Actualists hold that reality is at its core actual and that possible worlds should be understood in terms of actual entities, for example, as fictions or as sets of sentences.<ref name="Parent">{{cite web |last1=Parent |first1=Ted |title=Modal Metaphysics |url=https://iep.utm.edu/mod-meta/ |website=Internet Encyclopedia of Philosophy |access-date=5 January 2021}}</ref> Possibilists, on the other hand, assign to possible worlds the same ''fundamental ontological status'' as to the actual world. This is a form of [[Philosophy:Modal realism|modal realism]], holding that reality has ''irreducibly modal features''.<ref name="Parent"/> Another important issue in this field concerns the distinction between ''contingent'' and ''necessary beings''.<ref name="Sandkühler2"/> Contingent beings are beings whose existence is possible but not necessary. Necessary beings, on the other hand, could not have failed to exist.<ref>{{cite web |last1=Davidson |first1=Matthew |title=God and Other Necessary Beings |url=https://plato.stanford.edu/entries/god-necessary-being/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=5 January 2021 |date=2019}}</ref><ref>{{cite web |last1=Reichenbach |first1=Bruce |title=Cosmological Argument |url=https://plato.stanford.edu/entries/cosmological-argument/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=6 January 2021 |date=2019}}</ref> It has been suggested that this distinction is the highest division of being.<ref name="Sandkühler2"/><ref>{{cite web |title=Contingent |url=https://www.newadvent.org/cathen/04331a.htm |website=www.newadvent.org |publisher=CATHOLIC ENCYCLOPEDIA |access-date=5 January 2021}}</ref>

=== Substances ===
The category of ''substances'' has played a central role in many ontological theories throughout the history of philosophy.<ref name="kim">{{cite book |last1=Kim |first1=Jaegwon |last2=Sosa |first2=Ernest |last3=Rosenkrantz |first3=Gary S. |title=A Companion to Metaphysics |publisher=Wiley-Blackwell |url=https://philpapers.org/rec/KIMACT-9 |chapter=substance|year=1994 }}</ref><ref name="Robinson"/> "Substance" is a technical term within philosophy not to be confused with the more common usage in the sense of chemical substances like gold or sulfur. Various definitions have been given but among the most common features ascribed to substances in the philosophical sense is that they are ''particulars'' that are ''ontologically independent'': they are able to exist all by themselves.<ref name="kim"/><ref name="Borchert2"/> Being ontologically independent, substances can play the role of ''fundamental entities'' in the ''ontological hierarchy''.<ref name="Tahko"/><ref name="Robinson">{{cite web |last1=Robinson |first1=Howard |title=Substance |url=https://plato.stanford.edu/entries/substance/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref> If 'ontological independence' is defined as including ''causal independence'' then only self-caused entities, like Spinoza's God, can be substances. With a specifically ontological definition of 'independence', many everyday objects like books or cats may qualify as substances.<ref name="Borchert2"/><ref name="kim"/> Another defining feature often attributed to substances is their ability to ''undergo changes''. Changes involve something existing ''before'', ''during'' and ''after'' the change. They can be described in terms of a persisting substance gaining or losing properties, or of ''matter'' changing its ''form''.<ref name="kim"/> From this perspective, the ripening of a tomato may be described as a change in which the tomato loses its greenness and gains its redness. It is sometimes held that a substance can have a property in two ways: ''[[Philosophy:Essence|essentially]]'' and ''accidentally''. A substance can survive a change of ''accidental properties'' but it cannot lose its ''essential properties'', which constitute its nature.<ref name="Robinson"/><ref name="Robertson">{{cite web |last1=Robertson Ishii |first1=Teresa |last2=Atkins |first2=Philip |title=Essential vs. Accidental Properties |url=https://plato.stanford.edu/entries/essential-accidental/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref>

=== Properties and relations ===
{{main|Philosophy:Property|Philosophy:Relations}}
The category of ''properties'' consists of entities that can be exemplified by other entities, e.g. by substances.<ref name="Orilia">{{cite web |last1=Orilia |first1=Francesco |last2=Paolini Paoletti |first2=Michele |title=Properties |url=https://plato.stanford.edu/entries/properties/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref> Properties characterize their bearers, they express what their bearer is like.<ref name="Borchert2"/> For example, the red color and the round shape of an apple are properties of this apple. Various ways have been suggested concerning how to conceive properties themselves and their relation to substances.<ref name="Sandkühler2"/> The traditionally dominant view is that properties are universals that inhere in their bearers.<ref name="Borchert2"/> As universals, they can be shared by different substances. Nominalists, on the other hand, deny that universals exist.<ref name="MacLeod"/> Some nominalists try to account for properties in terms of resemblance relations or class membership.<ref name="Sandkühler2"/> Another alternative for nominalists is to conceptualize properties as simple particulars, so-called [[Philosophy:Trope#In metaphysics|tropes]].<ref name="Borchert2"/> This position entails that both the apple and its redness are particulars. Different apples may still exactly resemble each other concerning their color, but they do not share the same particular property on this view: the two color-tropes are ''numerically distinct''.<ref name="MacLeod"/> Another important question for any theory of properties is how to conceive the relation between a bearer and its properties.<ref name="Sandkühler2"/> Substratum theorists hold that there is some kind of substance, ''substratum'' or ''[[Philosophy:Substance theory#Bare particular|bare particular]]'' that acts as bearer.<ref name="Benovsky">{{cite journal |last1=Benovsky |first1=Jiri |title=The Bundle Theory and the Substratum Theory: Deadly Enemies or Twin Brothers? |journal=Philosophical Studies |date=2008 |volume=141 |issue=2 |pages=175–190 |doi=10.1007/s11098-007-9158-0 |s2cid=18712931 |url=https://philpapers.org/rec/BENTBT-2}}</ref> [[Philosophy:Bundle theory|Bundle theory]] is an alternative view that does away with a substratum altogether: objects are taken to be just a bundle of properties.<ref name="Robinson"/><ref>{{cite journal |last1=Rodriguez-Pereyra |first1=Gonzalo |title=The Bundle Theory is Compatible with Distinct but Indiscernible Particulars |journal=Analysis |date=2004 |volume=64 |issue=1 |pages=72–81 |doi=10.1093/analys/64.1.72 |url=https://philpapers.org/rec/ERETBT}}</ref> They are held together not by a substratum but by the so-called ''compresence-relation'' responsible for the bundling. Both substratum theory and bundle theory can be combined with conceptualizing properties as universals or as particulars.<ref name="Benovsky"/>

An important distinction among properties is between ''categorical'' and ''dispositional'' properties.<ref name="Borchert2"/><ref name="Kriegel"/> Categorical properties concern what something is like, e.g. what qualities it has. Dispositional properties, on the other hand, involve what powers something has, what it is able to do, even if it is not actually doing it.<ref name="Borchert2"/> For example, the shape of a sugar cube is a categorical property while its tendency to dissolve in water is a dispositional property. For many properties there is a lack of consensus as to how they should be classified, for example, whether colors are categorical or dispositional properties.<ref name="Choi"/><ref>{{cite web |last1=Rubenstein |first1=Eric M. |title=Color |url=https://iep.utm.edu/color/ |website=Internet Encyclopedia of Philosophy |access-date=7 January 2021}}</ref> [[Philosophy:Property#Categoricalism vs. dispositionalism|Categoricalism]] is the thesis that on a fundamental level there are only categorical properties, that dispositional properties are either non-existent or dependent on categorical properties. [[Philosophy:Property#Categoricalism vs. dispositionalism|Dispositionalism]] is the opposite theory, giving ontological primacy to dispositional properties.<ref name="Choi">{{cite web |last1=Choi |first1=Sungho |last2=Fara |first2=Michael |title=Dispositions |url=https://plato.stanford.edu/entries/dispositions/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2018}}</ref><ref name="Kriegel">{{cite journal |last1=Kriegel |first1=Uriah |title=Introverted Metaphysics: How We Get Our Grip on the Ultimate Nature of Objects, Properties, and Causation |journal=Metaphilosophy |date=2019 |volume=50 |issue=5 |pages=688–707 |doi=10.1111/meta.12391 |s2cid=211938090 |url=https://philpapers.org/rec/KRIIMH}}</ref> Between these two extremes, there are dualists who allow both categorical and dispositional properties in their ontology.<ref name="Orilia"/>

''Relations'' are ways in which things, the relata, stand to each other.<ref name="Borchert2"/><ref>{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=relations}}</ref> Relations are in many ways similar to properties in that both characterize the things they apply to. Properties are sometimes treated as a special case of relations involving only one relatum.<ref name="Orilia"/> Central for ontology is the distinction between ''internal'' and ''external'' relations.<ref name="MacBride">{{cite web |last1=MacBride |first1=Fraser |title=Relations |url=https://plato.stanford.edu/entries/relations/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=9 January 2021 |date=2020}}</ref> A relation is ''internal'' if it is fully determined by the features of its relata.<ref>{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=relations, the nature of}}</ref> For example, an apple and a tomato stand in the ''internal relation'' of [[Philosophy:Similarity|similarity]] to each other because they are both red.<ref>{{cite web |last1=Allen |first1=Sophie |title=Properties |url=https://iep.utm.edu/properties/ |website=Internet Encyclopedia of Philosophy |access-date=19 January 2021}}</ref> Some philosophers have inferred from this that internal relations do not have a proper ontological status since they can be reduced to intrinsic properties.<ref name="MacBride"/><ref>{{cite book |last1=Borchert |first1=Donald |title=Macmillan Encyclopedia of Philosophy, 2nd Edition |date=2006 |publisher=Macmillan |url=https://philpapers.org/rec/BORMEO |chapter=Relations, Internal and External}}</ref> ''External'' relations, on the other hand, are not fixed by the features of their relata. For example, a book stands in an ''external'' relation to a table by lying on top of it. But this is not determined by the book's or the table's features like their color, their shape, etc.<ref name="MacBride"/>

=== States of affairs and events ===
[[Philosophy:State of affairs|States of affairs]] are complex entities, in contrast to substances and properties, which are usually conceived as simple.<ref name="Borchert2"/><ref>{{cite book |last1=Meinertsen |first1=Bo R. |title=Metaphysics of States of Affairs: Truthmaking, Universals, and a Farewell to Bradley's Regress |year=2018 |publisher=Springer Singapore |page=1 |url=https://philpapers.org/rec/MEIMOS-2}}</ref> Complex entities are built up from or constituted by other entities. Atomic states of affairs are constituted by one particular and one property exemplified by this particular.<ref name="Sandkühler2"/><ref name="Textor">{{cite web |last1=Textor |first1=Mark |title=States of Affairs |url=https://plato.stanford.edu/entries/states-of-affairs/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=9 January 2021 |date=2020}}</ref> For example, the state of affairs that Socrates is wise is constituted by the particular "Socrates" and the property "wise". Relational states of affairs involve several particulars and a relation connecting them. States of affairs that ''obtain'' are also referred to as [[Philosophy:Fact#In philosophy|facts]].<ref name="Textor"/> It is controversial which ontological status should be ascribed to states of affairs that do not obtain.<ref name="Sandkühler2"/> States of affairs have been prominent in 20th-century ontology as various theories were proposed to describe the world as composed of states of affairs.<ref name="Borchert2"/><ref>{{cite book |last1=Wittgenstein |first1=Ludwig |last2=Colombo |first2=G. C. M. |last3=Russell |first3=Bertrand |title=Tractatus Logico-Philosophicus |date=1922 |publisher=Fratelli Bocca |url=https://philpapers.org/rec/WITTL-14}}</ref><ref>{{cite book |last1=Armstrong |first1=D. M. |title=A World of States of Affairs |date=1996 |publisher=Cambridge University Press |url=https://philpapers.org/rec/ARMAWO-3}}</ref> It is often held that states of affairs play the role of truthmakers: judgments or assertions are true because the corresponding state of affairs obtains.<ref name="Textor"/><ref>{{cite web |last1=Asay |first1=Jamin |title=Truthmaker Theory |url=https://iep.utm.edu/truth-ma/ |website=Internet Encyclopedia of Philosophy |access-date=9 January 2021}}</ref>

[[Philosophy:Event|Events]] take place in time, they are sometimes thought of as involving a change in the form of acquiring or losing a property, like the lawn's becoming dry.<ref name="Honderich2"/> But on a liberal view, the retaining of a property without any change may also count as an event, e.g. the lawn's staying wet.<ref name="Honderich2">{{cite book |last1=Honderich |first1=Ted |title=The Oxford Companion to Philosophy |date=2005 |publisher=Oxford University Press |url=https://philpapers.org/rec/HONTOC-2 |chapter=events}}</ref><ref name="Kim2"/> Some philosophers see events as universals that can repeat at different times, but the more dominant view is that events are particulars and therefore non-repeatable.<ref name="Kim2">{{cite book |last1=Kim |first1=Jaegwon |last2=Sosa |first2=Ernest |last3=Rosenkrantz |first3=Gary S. |title=A Companion to Metaphysics |publisher=Wiley-Blackwell |url=https://philpapers.org/rec/KIMACT-9 |chapter=event theory|year=1994 }}</ref> Some events are complex in that they are composed of a sequence of events, often referred to as a process.<ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=processes}}</ref> But even simple events can be conceived as complex entities involving an object, a time and the property exemplified by the object at this time.<ref>{{cite book |last1=Audi |first1=Robert |title=The Cambridge Dictionary of Philosophy |publisher=Cambridge University Press |url=https://philpapers.org/rec/AUDTCD-2 |chapter=event|year=1999 }}</ref><ref>{{cite web |last1=Schneider |first1=Susan |title=Events |url=https://iep.utm.edu/events/ |website=Internet Encyclopedia of Philosophy}}</ref> So-called [[Philosophy:Process philosophy|process philosophy]] or [[Process ontology|process ontology]] ascribes ontological primacy to changes and processes as opposed to the emphasis on static being in the traditionally dominant substance metaphysics.<ref>{{cite web |last1=Seibt |first1=Johanna |title=Process Philosophy |url=https://plato.stanford.edu/entries/process-philosophy/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=9 January 2021 |date=2020}}</ref><ref>{{cite web |last1=Hustwit |first1=J. R. |title=Process Philosophy |url=https://iep.utm.edu/processp/ |website=Internet Encyclopedia of Philosophy |access-date=9 January 2021}}</ref>

===Reality of things===

The word 'real' is derived from the Latin word ''res'', which is often translated as 'thing'. The word 'thing' is often used in ontological discourse as if it had a presupposed meaning, not needing an explicit philosophical definition because it belongs to ordinary language. Nevertheless, what is a thing and what is real or substantial are concerns of ontology.<ref name = "Erices"/><ref>Thomasson, A. L. (2007). ''Ordinary Objects'', Oxford University Press, New York, {{ISBN|978-0195319910}}, p. 7: "I argue that while there are various ways of addressing questions about what 'things' exist, ..."</ref><ref>{{cite book |last1=Sandkühler |first1=Hans Jörg |url=https://meiner.de/enzyklopadie-philosophie.html |title=Enzyklopädie Philosophie |date=2010 |publisher=Meiner |language=de |section=Zweifel, methodischer: 2 Zur Begriffs- und Problemgeschichte. ›reale Dinge (entia realia)‹ |access-date=2020-12-16 |archive-url=https://web.archive.org/web/20210311040207/https://meiner.de/enzyklopadie-philosophie.html |archive-date=2021-03-11 |url-status=dead}}</ref><ref>Eddington, A.S. (1928). ''The Nature of the Physical World: The Gifford Lectures 1927'', Macmillan, London, England and New York, reprinted 1929 p. ix: "... it is the distinctive characteristic of a "thing" to have this substantiality".</ref><ref name = "Isham"/> Different views are held about this. Plato proposed that underlying, and constituting the real basis of, the concretely experienced world are '[[Philosophy:Theory of forms|forms]]' or 'ideas', which today are generally regarded as high abstractions. In earlier days, philosophers used the term '[[Philosophy:Platonic realism|realism]]' to refer to Plato's belief that his 'forms' are 'real'; nowadays, the term 'realism' often has an almost opposite meaning, so that Plato's belief is sometimes called '[[Philosophy:Idealism|idealism]]'.<ref>Hale, B. (2009). "Realism and antirealism about abstract entities", pp. 65–73 in Jaegwon Kim, Ernest Sosa, and Gary S. Rosenkrantz, ''A Companion to Metaphysics'', 2nd ed., Wiley–Blackwell, Chichester, UK, {{ISBN|978-1405152983}}.1.</ref> Philosophers debate whether entities such as tables and chairs, lions and tigers, philosophical doctrines, numbers, truth, and beauty, are to be regarded as 'things', or as something or nothing 'real'.

==Types of ontologies==
Ontological theories can be divided into various types according to their theoretical commitments. Particular ontological theories or types of theories are often referred to as "ontologies" (singular or plural). This usage contrasts with the meaning of "ontology" (only singular) as a branch of philosophy: the ''science of being'' in general.<ref>{{cite journal |last1=Simons |first1=Peter |title=Ontology Meets Ontologies: Philosophers as Healers |journal=Metascience |date=2009 |volume=18 |issue=3 |pages=469–473 |doi=10.1007/s11016-009-9308-4 |s2cid=170379747 |url=https://philpapers.org/rec/SMIOMO}}</ref><ref>{{cite web |title=Ontology |url=https://www.merriam-webster.com/dictionary/ontology |website=www.merriam-webster.com |access-date=16 February 2021 |language=en}}</ref>

=== Flat vs polycategorical vs hierarchical ===
One way to divide ontologies is by the number of basic categories they use. ''Monocategorical'' or ''one category ontologies'' hold that there is only one basic category while ''polycategorical ontologies'' imply that there are several distinct basic categories.<ref name="Inwagen"/><ref>{{cite book |last1=Paul |first1=L. A. |title=Being, Freedom, and Method: Themes From the Philosophy of Peter van Inwagen |date=2017 |publisher=Oxford University Press |url=https://philpapers.org/rec/PAUAOC-4 |chapter=A One Category Ontology}}</ref><ref>{{cite web |last1=Thomasson |first1=Amie |title=Categories |url=https://plato.stanford.edu/entries/categories/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=27 December 2020 |date=2019}}</ref> Another way to divide ontologies is through the notion of ontological hierarchy. Hierarchical ontologies assert that some entities exist on a more fundamental level and that other entities depend on them. Flat ontologies, on the other hand, deny such a privileged status to any entities.<ref>{{cite book |last1=Brassier |first1=Ray |title=Under Influence – Philosophical Festival Drift (2014) |date=2015 |publisher=Omnia |pages=64–80 |url=https://philpapers.org/rec/BRADAF |chapter=Deleveling: Against 'Flat Ontologies'}}</ref><ref>{{cite web |last1=Epstein |first1=Brian |title=Social Ontology |url=https://plato.stanford.edu/entries/social-ontology/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=27 November 2020 |date=2018}}</ref> Jonathan Schaffer provides an overview of these positions by distinguishing between ''flat'' ontologies (non-hierarchical), ''sorted'' ontologies (polycategorical non-hierarchical) and ''ordered'' ontologies (polycategorical hierarchical).<ref name=Schaffer>{{cite book |author=[[Biography:Jonathan Schaffer|Jonathan Schaffer]] |chapter=On What Grounds What Metametaphysics |title=Metametaphysics |chapter-url=http://www.jonathanschaffer.org/grounds.pdf |editor=Chalmers |editor2=Manley |editor3=Wasserman |isbn=978-0199546046 |publisher=Oxford University Press |year=2009 |pages=347–383 }}</ref>

''Flat'' ontologies are only interested in the difference between existence and non-existence. They are ''flat'' because each flat ontology can be represented by a simple set containing all the entities to which this ontology is committed. An influential exposition<ref>{{cite journal |last1=Quine |first1=Willard V. |title=On What There Is |journal=Review of Metaphysics |date=1948 |volume=2 |issue=1 |pages=21–38 |url=https://philpapers.org/rec/QUIOWT-11}}</ref> of this approach comes from [[Biography:Willard Van Orman Quine|Willard Van Orman Quine]], which is why it has been referred to as the [[Philosophy:Meta-ontology#Quinean approach|Quinean]] approach to [[Philosophy:Meta-ontology|meta-ontology]].<ref name=Schaffer/><ref>{{cite web |last1=Bricker |first1=Phillip |title=Ontological Commitment |url=https://plato.stanford.edu/entries/ontological-commitment/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2016}}</ref> This outlook does not deny that the existing entities can be further subdivided and may stand in various relations to each other. These issues are questions for the more specific sciences, but they do not belong to ontology in the Quinean sense.

''Polycategorical'' ontologies are concerned with the categories of being. Each polycategorical ontology posits a number of categories. These categories are exclusive and exhaustive: every existing entity belongs to exactly one category.<ref name=Schaffer/> A recent example of a polycategorical ontology is E.J. Lowe's four-category-ontology.<ref name=Lowe>{{cite book |last1=Lowe |first1=E. J. |last2=Lowe |first2=Edward Joseph |title=The Four-Category Ontology: A Metaphysical Foundation for Natural Science |publisher=Clarendon Press |isbn=978-0199254392 |url=https://books.google.com/books?id=s7ISDAAAQBAJ |language=en |chapter=2. The Four-Category Ontology and its Rivals|year=2006 }}</ref> The four categories are object, kind, mode and attribute. The fourfold structure is based on two distinctions. The first distinction is between substantial entities (objects and kinds) and non-substantial entities (modes and attributes). The second distinction is between particular entities (objects and modes) and universal entities (kinds and attributes). Reality is built up through the interplay of entities belonging to different categories: particular entities instantiate universal entities, and non-substantial entities characterize substantial entities.<ref name=Lowe/><ref>{{cite web |last1=Miller |first1=J. T. M. |title=Lowe, Edward Jonathan |url=https://iep.utm.edu/lowe-ej/ |website=Internet Encyclopedia of Philosophy}}</ref>

''Hierarchical'' ontologies are interested in the degree of fundamentality of the entities they posit. Their main goal is to figure out which entities are fundamental and how the non-fundamental entities depend on them. The concept of fundamentality is usually defined in terms of [[Philosophy:Grounding (metaphysics)|metaphysical grounding]].<ref>{{cite web |last1=Tahko |first1=Tuomas E. |title=Fundamentality |url=https://plato.stanford.edu/entries/fundamentality/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2018}}</ref> Fundamental entities are different from non-fundamental entities because they are not grounded in other entities.<ref name=Schaffer/><ref>{{cite journal |last1=Mehta |first1=Neil |title=Can Grounding Characterize Fundamentality? |journal=Analysis |date=2017 |volume=77 |issue=1 |pages=74–79 |doi=10.1093/analys/anx044 |url=https://philpapers.org/rec/NEICGC}}</ref> For example, it is sometimes held that elementary particles are more fundamental than the macroscopic objects (like chairs and tables) they compose. This is a claim about the grounding-relation between microscopic and macroscopic objects. Schaffer's priority monism is a recent form of a hierarchical ontology. He holds that on the most fundamental level there exists only one thing: the world as a whole. This thesis does not deny our common-sense intuition that the distinct objects we encounter in our everyday affairs like cars or other people exist. It only denies that these objects have the most fundamental form of existence.<ref>{{cite journal |last1=Schaffer |first1=Jonathan |title=Monism: The Priority of the Whole |journal=The Philosophical Review |date=1 January 2010 |volume=119 |issue=1 |pages=31–76 |doi=10.1215/00318108-2009-025 |url=https://read.dukeupress.edu/the-philosophical-review/article-abstract/119/1/31/2871/Monism-The-Priority-of-the-Whole |language=en |issn=0031-8108}}</ref> An example of a ''hierarchical'' ontology in [[Philosophy:Continental philosophy|continental philosophy]] comes from [[Biography:Nicolai Hartmann|Nicolai Hartmann]]. He asserts that reality is made up of four [[Integrative level|levels]]: the inanimate, the biological, the psychological and the spiritual.<ref name="Poli">{{cite web |last1=Poli |first1=Roberto |title=Nicolai Hartmann |url=https://plato.stanford.edu/entries/nicolai-hartmann/#LeveReal |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2017}}</ref> These levels form a hierarchy in the sense that the higher levels depend on the lower levels while the lower levels are indifferent to the higher levels.<ref name="Hartmann">{{cite book |last1=Hartmann |first1=Nicolai |title=New Ways of Ontology |publisher=Transaction Publishers |isbn=978-1412847049 |url=https://books.google.com/books?id=4PyJfVZn-rcC |language=en |chapter=9 Dependence and Autonomy in the Hierarchy of Strata|year= 2012 }}</ref>

===Thing ontologies vs fact ontologies===
''Thing ontologies'' and ''fact ontologies'' are one-category-ontologies: they both hold that all fundamental entities belong to the same category. They disagree on whether this category is the category of things or of facts.<ref>{{cite journal |last1=Bergmann |first1=Gustav |title=Ineffability, Ontology, and Method |journal=Philosophical Review |date=1960 |volume=69 |issue=1 |pages=18–40 |doi=10.2307/2182265 |jstor=2182265 |url=https://philpapers.org/rec/BERIOA}}</ref><ref>{{cite journal |last1=Rosenkrantz |first1=Gary S. |title=Of Facts and Things |journal=International Journal of Philosophical Studies |date=2018 |volume=26 |issue=5 |pages=679–700 |doi=10.1080/09672559.2018.1542277 |s2cid=149893677 |url=https://philpapers.org/rec/ROSOFA}}</ref><ref>{{cite book |last1=Esfeld |first1=Michael |title=The Routledge Handbook of Metametaphysics |url=https://philpapers.org/rec/ESFTAN |chapter='Thing' and 'Non-Thing' Ontologies}}</ref> A slogan for fact ontologies comes from [[Biography:Ludwig Wittgenstein|Ludwig Wittgenstein]]: "The world is the totality of facts, not of things".<ref>{{cite book |last1=Wittgenstein |first1=Ludwig |title=Tractatus Logico-philosophicus |date=2001 |publisher=Routledge |page=5}}</ref>

One difficulty in characterizing this dispute is to elucidate what things and facts are, and how they differ from each other. Things are commonly contrasted with the properties and relations they instantiate.<ref name=Bradley>{{cite web |last1=Rettler |first1=Bradley |last2=Bailey |first2=Andrew M. |title=Object |url=https://plato.stanford.edu/entries/object/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2017}}</ref> Facts, on the other hand, are often characterized as having these things and the properties/relations as their constituents.<ref name="Armstrong">{{cite book |last1=Armstrong |first1=D. M. |url=https://books.google.com/books?id=pbGI46EoQKEC |title=Sketch for a Systematic Metaphysics |date=29 July 2010 |publisher=Oxford University Press |isbn=978-0191615429 |location=Oxford, England |language=en-uk |chapter=4. States of Affairs}}</ref> This is reflected in a rough linguistic characterization of this difference where the subjects and objects of an assertion refer to things while the assertion as a whole refers to a fact.<ref>{{cite journal |last1=Skyrms |first1=Brian |title=Tractarian Nominalism |journal=Philosophical Studies |date=1981 |volume=40 |issue=2 |pages=199–206 |doi=10.1007/BF00353791 |s2cid=170360466 |url=https://philpapers.org/rec/SKYTN}}</ref>

[[Philosophy:Reism|Reism]] is one form of thing ontology.<ref name="Woleński">{{Citation |last=Woleński |first=Jan |title=Reism |date=2020 |url=https://plato.stanford.edu/archives/sum2020/entries/reism/ |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |access-date=2021-07-28 |edition=Summer 2020 |publisher=Metaphysics Research Lab, Stanford University}}.</ref> [[Biography:Franz Brentano|Franz Brentano]] developed a version of reism in his later philosophy. He held that only concrete particular things exist. Things can exist in two forms: either as spatio-temporal bodies or as temporal souls. Brentano was aware of the fact that many common-sense expressions seem to refer to entities that do not have a place in his ontology, like properties or intentional objects. This is why he developed a method to paraphrase these expressions in order to avoid these ontological commitments.<ref name="Woleński"/>

D. M. Armstrong is a well-known defender of fact ontology. He and his followers refer to facts as states of affairs.<ref name=Armstrong /> States of affairs are the basic building blocks of his ontology: they have particulars and universals as their constituents but they are primary in relation to particulars and universals. States of affairs have ontologically independent existence while "[u]npropertied particulars and uninstantiated universals are false abstractions".<ref name=Armstrong />

===Constituent ontologies vs blob theories===
''Constituent ontologies'' and ''blob theories'', sometimes referred to as ''relational ontologies'', are concerned with the internal structure of objects. Constituent ontologies hold that objects have an internal structure made up of constituents. This is denied by opposing theories, which contend that objects are homogeneous, internally indifferentiable "blobs".<ref name=Bradley/><ref name="Inwagen">{{cite journal |last1=Inwagen |first1=Peter van |title=Relational Vs. Constituent Ontologies |journal=Philosophical Perspectives |date=2011 |volume=25 |issue=1 |pages=389–405 |doi=10.1111/j.1520-8583.2011.00221.x |url=https://philpapers.org/rec/VANRVC}}</ref><ref>{{cite book |last1=Vallicella |first1=William F. |title=A Paradigm Theory of Existence: Onto-Theology Vindicated |publisher=Kluwer Academic Publishers |page=88 |url=https://philpapers.org/rec/VALAPT-2 |chapter=III The 'No Difference' Theory|year=2002 }}</ref>

[[Philosophy:Bundle theory|Bundle theories]] are examples of constituent ontologies. Bundle theorists assert that an object is nothing but the properties it "has". On this account, a regular apple could be characterized as a bundle of redness, roundness, sweetness, etc. Defenders of bundle theory disagree on the nature of the bundled properties. Some affirm that these properties are universals while others contend that they are particulars, so-called "tropes".<ref name=Bradley/><ref>{{cite web |last1=Robinson |first1=Howard |title=Substance |url=https://plato.stanford.edu/entries/substance/#BundTheoVersSubsThinPart |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref>

Class [[Philosophy:Nominalism|nominalism]], on the other hand, is a form of blob theory. Class nominalists hold that properties are classes of things. To instantiate a property is merely to be a member of the corresponding class. So properties are not constituents of the objects that have them.<ref name=Bradley/><ref>{{cite web |last1=Rodriguez-Pereyra |first1=Gonzalo |title=Nominalism in Metaphysics |url=https://plato.stanford.edu/entries/nominalism-metaphysics/#NomAboUni |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2019}}</ref>

===Information science and natural sciences===
In [[Ontology (information science)|information science]] ontologies are classified in various ways, using criteria such as the degree of abstraction and field of application:<ref name=Petrov />

# ''[[Upper ontology]]'': concepts supporting development of an ontology, [[Philosophy:Meta-ontology|meta-ontology]].
# ''[[Ontology (information science)|Domain ontology]]'': concepts relevant to a particular topic, [[Philosophy:Domain of discourse|domain of discourse]], or area of interest, for example, to information technology or to computer languages, or to particular branches of science.
# ''Interface ontology'': concepts relevant to the juncture of two disciplines.
# ''[[Process ontology]]'': inputs, outputs, constraints, sequencing information, involved in business or engineering processes.
In the [[Medicine:Biomedical sciences|biomedical sciences]], ontologies have been used to create terminologies for various aspects of living organism or medical applications. A prominent example is the [[Biology:Gene Ontology|gene ontology]], but many other ontologies exist, e.g. for anatomical terms or physiology.<ref name=":1">{{Cite journal|last1=Bard|first1=Jonathan B. L.|last2=Rhee|first2=Seung Y.|date=March 2004|title=Ontologies in biology: design, applications and future challenges|url=https://www.nature.com/articles/nrg1295|journal=Nature Reviews Genetics|language=en|volume=5|issue=3|pages=213–222|doi=10.1038/nrg1295|pmid=14970823 |s2cid=10618089 |issn=1471-0064}}</ref> Standards have been established to maintain and organize biological ontologies under the [[Organization:OBO Foundry|OBO]] (Open Biological Ontologies) project.<ref name=":1" />

== History ==
=== Ancient Greek ===
[[File:Parmenides.jpg|thumb|upright|Parmenides was among the first to propose an ontological characterization of the fundamental nature of reality.]]
In the [[Philosophy:Ancient Greek philosophy|Greek philosophical tradition]], [[Biography:Parmenides|Parmenides]] was among the first to propose an ontological characterization of the fundamental nature of existence. In the prologue (or [[Biography:Parmenides#Proem|''proem'']]) to ''On Nature'', he describes two views of [[Philosophy:Existence|existence]]. Initially, nothing comes from nothing, thus [[Philosophy:Existence|existence]] is [[Philosophy:Monism|eternal]]. This posits that existence is what may be conceived of by thought, created, or possessed. Hence, there may be neither void nor vacuum; and true reality neither may come into being nor vanish from existence. Rather, the entirety of creation is eternal, uniform, and immutable, though not infinite (Parmenides characterized its shape as that of a perfect sphere). Parmenides thus posits that change, as perceived in everyday experience, is illusory.

Opposite to the Eleatic [[Philosophy:Monism|monism]] of [[Biography:Parmenides|Parmenides]] is the [[Philosophy:Pluralism|pluralistic]] conception of [[Philosophy:Being|being]]. In the 5th century BC, [[Biography:Anaxagoras|Anaxagoras]] and [[Biography:Leucippus|Leucippus]] replaced<ref>{{Cite book |last=Graham |first=Daniel W. |url=https://press.princeton.edu/books/hardcover/9780691125404/explaining-the-cosmos |title=Explaining the Cosmos |date=2006-08-06 |publisher=Princeton University Press |isbn=978-0-691-12540-4 |language=en}}</ref> the reality of ''being'' (unique and unchanging) with that of [[Philosophy:Becoming|''becoming'']], therefore by a more fundamental and elementary ''[[Philosophy:Ontic|ontic]]'' plurality. This thesis originated in the Hellenic world, stated in two different ways by Anaxagoras and by Leucippus. The first theory dealt with "seeds" (which Aristotle referred to as "''homeomeries''") of the various substances. The second was the [[Atom (order theory)|atomistic]] theory,<ref>{{cite web |url= http://plato.stanford.edu/entries/atomism-ancient/ | title= Ancient Atomism (Stanford Encyclopedia of Philosophy) | publisher= Plato.stanford.edu | access-date= 2010-02-21}}</ref> which dealt with reality as based on the [[Physics:Vacuum|vacuum]], the atoms and their intrinsic movement in it.<ref>Lloyd, G. E. R. (2006). Leucippus and Democritus. In D. M. Borchert (Ed.), ''Encyclopedia of philosophy'' (2nd ed., Vol. 5, pp. 297–303). Macmillan Reference USA.</ref>

The materialist ''[[Philosophy:Atomism|atomism]]'' proposed by Leucippus was [[Indeterminism|indeterminist]], but [[Biography:Democritus|Democritus]] ({{circa}} 460 – {{circa}} 370 BC) subsequently developed it in a [[Determinism|deterministic]] way. Later (4th century BC), [[Biography:Epicurus|Epicurus]] took the original ''atomism'' again as indeterministic. He saw reality as composed of an infinity of indivisible, unchangeable corpuscles or [[Physics:Atom|atom]]s (from the Greek ''atomon'', lit. 'uncuttable'), but he gives weight to characterize atoms whereas for Leucippus they are characterized by a "figure", an "order" and a "position" in the cosmos.<ref>Aristotle, ''Metaphysics'', I, 4, p. 985.</ref> Atoms are, besides, creating the whole with the intrinsic movement in the ''vacuum'', producing the diverse flux of being. Their movement is influenced by the ''parenklisis'' ([[Biography:Lucretius|Lucretius]] names it ''[[Philosophy:Clinamen|clinamen]]'') and that is determined by [[Randomness|chance]]. These ideas foreshadowed the understanding of [[Physics:Physics|traditional physics]] until the advent of 20th-century theories on the nature of atoms.<ref>
{{cite book
 | year = 2007
 | editor1-last = Lawson
 | editor1-first = Clive
 | editor2-last = Latsis
 | editor2-first = John Spiro
 | editor3-last = Martins
 | editor3-first = Nuno
 | title = Contributions to Social Ontology
 | url = https://books.google.com/books?id=j5YEviKRlyIC
 | series = Routledge Studies in Critical Realism
 | location = London
 | publisher = Routledge
 | publication-date = 2013
 | isbn = 978-1136016066
 | access-date = 3 Mar 2019
}}

[[Biography:Plato|Plato]] developed the distinction between true reality and illusion, in arguing that what is real are eternal and unchanging forms or ideas (a precursor to [[Philosophy:Universal (metaphysics)|''universals'']]), of which things experienced in sensation are at best merely copies, and real only in so far as they copy ("partake of") such forms. In general, Plato presumes that all nouns (e.g., "beauty") refer to real entities, whether sensible bodies or insensible forms. Hence, in ''The Sophist'', Plato argues that ''being'' is a ''form'' in which all existent things participate and which they have in common (though it is unclear whether "Being" is intended in the sense of [[Philosophy:Existence|existence]], copula, or [[Philosophy:Identity|identity]]); and argues, against [[Biography:Parmenides|Parmenides]], that forms must exist not only of ''being'', but also of [[Negation]] and of non-being (or ''Difference'').{{citation needed |date= September 2017}}

In his [[Philosophy:Categories (Aristotle)|''Categories'']], Aristotle (384–322 BCE) identifies ten possible kinds of things that may be the subject or the predicate of a proposition. For Aristotle there are four different ontological dimensions:<ref>{{Cite encyclopedia |last=Studtmann |first=Paul |date=2007-09-07 |title=Aristotle's Categories |encyclopedia=Stanford Encyclopedia of Philosophy |url=https://stanford.library.sydney.edu.au/entries/aristotle-categories/#FouFolDiv}}</ref>

# according to the various categories or ways of addressing a being as such
# according to its truth or falsity (e.g. fake gold, counterfeit money)
# whether it exists in and of itself or simply 'comes along' by accident
# according to its potency, movement (energy) or finished presence (''Metaphysics'' Book Theta).

===Hindu philosophy===
Ontology features in the [[Philosophy:Samkhya|Samkhya]] school of [[Philosophy:Hindu philosophy|Hindu philosophy]] from the first millennium BCE.<ref>Larson, G.J., R.S. Bhattacharya, and K. Potter, eds. 2014. "Samkhya." pp. 3–11 in ''The Encyclopedia of Indian Philosophies'' 4. Princeton University Press. {{ISBN|978-0691604411}}.</ref> Samkhya philosophy regards the universe as consisting of two independent realities: puruṣa (pure, contentless consciousness) and [[Philosophy:Prakṛti|prakṛti]] (matter). The substance dualism between puruṣa and prakṛti is similar but not identical to the substance dualism between mind and body that, following the works of Descartes, has been central to many disputes in the Western philosophical tradition.<ref name="Schweizer"/>{{rp|845}} Samkhya sees the mind as being the subtle part of prakṛti. It is made up of three faculties: the sense mind (manas), the intellect ([[Philosophy:Buddhi|buddhi]]), and the ego ([[Philosophy:Ahamkara|ahaṁkāra]]). These faculties perform various functions but are by themselves unable to produce consciousness, which belongs to a distinct ontological category and for which puruṣa alone is responsible.<ref>{{cite web |last1=Ruzsa |first1=Ferenc |title=Sankhya |url=https://iep.utm.edu/sankhya/ |website=Internet Encyclopedia of Philosophy |access-date=11 December 2020}}</ref><ref name="Schweizer">{{cite journal |last1=Schweizer |first1=Paul |title=Mind/Consciousness Dualism in Sankhya–Yoga Philosophy |journal=Philosophy and Phenomenological Research |date=1993 |volume=53 |issue=4 |pages=845–859 |doi=10.2307/2108256 |jstor=2108256 |url=https://philpapers.org/rec/SCHMDI}}</ref> The Yoga school agrees with Samkhya philosophy on the fundamental dualism between puruṣa and prakṛti but it differs from Samkhya's atheistic position by incorporating the concept of a "personal, yet essentially inactive, deity" or "personal god" ([[Philosophy:Ishvara|Ishvara]]).<ref>Mikel Burley (2012), ''Classical Samkhya and Yoga – An Indian Metaphysics of Experience'', Routledge, {{ISBN|978-0415648875}}, pp. 39–41.</ref><ref name="lpfl">Lloyd Pflueger, ''Person Purity and Power in Yogasutra, in Theory and Practice of Yoga'' (Editor: Knut Jacobsen), Motilal Banarsidass, {{ISBN|978-8120832329}}, pp. 38–39.</ref><ref>Kovoor T. Behanan (2002), ''Yoga: Its Scientific Basis'', Dover, {{ISBN|978-0486417929}}, pp. 56–58.</ref><ref>{{cite web |last1=Bryant |first1=Edwin |title=Yoga Sutras of Patanjali |url=https://iep.utm.edu/yoga/ |website=Internet Encyclopedia of Philosophy |access-date=11 December 2020}}</ref> These two schools stand in contrast to [[Philosophy:Advaita Vedanta|Advaita Vedanta]], which adheres to non-duality by revealing that the apparent plurality of things is an illusion (Maya) hiding the true oneness of reality at its most fundamental level ([[Philosophy:Brahman|Brahman]]).<ref>{{cite web |last1=Menon |first1=Sangeetha |title=Vedanta, Advaita |url=https://iep.utm.edu/adv-veda/ |website=Internet Encyclopedia of Philosophy |access-date=11 December 2020}}</ref><ref>{{cite web |last1=Ranganathan |first1=Shyam |title=Hindu Philosophy |url=https://iep.utm.edu/hindu-ph/ |website=Internet Encyclopedia of Philosophy |access-date=11 December 2020}}</ref>

=== Medieval ===
''Medieval ontology'' was strongly influenced by Aristotle's teachings. The thinkers of this period often relied on Aristotelian categories like ''substance'', ''act and potency'' or ''matter and form'' to formulate their own theories. Important ontologists in this epoch include Avicenna, Thomas Aquinas, [[Biography:Duns Scotus|Duns Scotus]] and [[Biography:William of Ockham|William of Ockham]].<ref>{{cite web |last1=Gracia |first1=Jorge |last2=Newton |first2=Lloyd |title=Medieval Theories of the Categories |url=https://plato.stanford.edu/entries/medieval-categories/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=1 January 2021 |date=2016}}</ref><ref>{{cite web |last1=Brower |first1=Jeffrey |title=Medieval Theories of Relations |url=https://plato.stanford.edu/entries/relations-medieval/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=1 January 2021 |date=2018}}</ref><ref name="Dahlstrom"/>

According to [[Biography:Avicenna|Avicenna]]'s interpretation of Greek Aristotelian and Platonist ontological doctrines in medieval [[Philosophy:Metaphysics|metaphysics]], being is either necessary, contingent ''qua'' possible, or impossible. Necessary being is that which cannot but be, since its non-being would entail a contradiction. Contingent ''qua'' possible being is neither necessary nor impossible for it to be or not to be. It is ontologically neutral, and is brought from potential existing into actual existence by way of a cause that is external to its essence. Its being is borrowed – unlike the necessary existent, which is self-subsisting and impossible not to be. As for the impossible, it necessarily does not exist, and the affirmation of its being would involve a contradiction.<ref>
[[Biography:Nader El-Bizri|Nader El-Bizri]], 'Ibn Sina and [[Philosophy:Essentialism|Essentialism]], Review of [[Philosophy:Metaphysics|Metaphysics]], Vol. 54 (2001), pp. 753–778.
</ref>

Fundamental to [[Biography:Thomas Aquinas|Thomas Aquinas]]'s ontology is his distinction between [[Philosophy:Essence|essence]] and [[Philosophy:Existence|existence]]: all entities are conceived as composites of essence and existence.<ref name="Kerr">{{cite web |last1=Kerr |first1=Gaven |title=Aquinas: Metaphysics |url=https://iep.utm.edu/aq-meta/ |website=Internet Encyclopedia of Philosophy |access-date=18 December 2020}}</ref><ref>{{cite web |last1=Brown |first1=Christopher M. |title=Thomas Aquinas |url=https://iep.utm.edu/aquinas/ |website=Internet Encyclopedia of Philosophy |access-date=18 December 2020}}</ref><ref>{{cite web |last1=Magee |first1=Joseph |title=Ontology |url=https://aquinasonline.com/ontology/ |website=Thomistic Philosophy Page |access-date=18 December 2020 |language=en |date=9 February 2020}}</ref> The essence of a thing is what this thing is like, it signifies the definition of this thing.<ref>{{cite web |last1=Magee |first1=Joseph |title=Essence and Existence |url=https://aquinasonline.com/essence-and-existence/ |website=Thomistic Philosophy Page |access-date=18 December 2020 |language=en |date=4 February 2020}}</ref> God has a special status since He is the only entity whose essence is identical to its existence. But for all other, finite entities there is a ''real distinction'' between essence and existence.<ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Aquinas, Thomas}}</ref> This distinction shows itself, for example, in our ability to understand the essence of something without knowing about its existence.<ref>{{cite book |last1=Borchert |first1=Donald |title=Macmillan Encyclopedia of Philosophy, 2nd Edition |date=2006 |publisher=Macmillan |url=https://philpapers.org/rec/MONMEO-3 |chapter=Thomas Aquinas, St.}}</ref> Aquinas conceives of existence as an ''act of being'' that actualizes the potency given by the essence. Different things have different essences, which impose different limits on the corresponding ''act of being''.<ref name="Kerr"/> The paradigm examples of essence-existence-composites are material substances like cats or trees. Aquinas incorporates Aristotle's distinction between matter and form by holding that the essence of ''material'' things, as opposed to the essence of ''immaterial'' things like angels, is the composition of their matter and form.<ref name="Kerr"/><ref>{{cite web |last1=McInerny |first1=Ralph |last2=O'Callaghan |first2=John |title=Saint Thomas Aquinas |url=https://plato.stanford.edu/entries/aquinas/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=18 December 2020 |date=2018}}</ref> So, for example, the essence of a marble statue would be the composition of the marble (its matter) and the shape it has (its form). Form is universal since substances made of different matter can have the same form. The forms of a substance may be divided into substantial and accidental forms. A substance can survive a change of an accidental form but ceases to exist upon a change of a substantial form.<ref name="Kerr"/>

=== Modern ===
Ontology is increasingly seen as a separate domain of philosophy in the ''modern period''.<ref name="Dahlstrom"/><ref>{{cite book |last1=Jaroszyski |first1=Piotr |title=Metaphysics or Ontology? |date=2018 |publisher=Brill |url=https://philpapers.org/rec/JARMOO |chapter=Summary of Part 2}}</ref> Many ontological theories of this period were rationalistic in the sense that they saw ontology largely as a deductive discipline that starts from a small set of first principles or axioms, a position best exemplified by Baruch Spinoza and Christian Wolff. This rationalism in metaphysics and ontology was strongly opposed by [[Biography:Immanuel Kant|Immanuel Kant]], who insisted that many claims arrived at this way are to be dismissed since they go beyond any possible experience that could justify them.<ref name="Borchert"/><ref name="Sandkühler"/>

[[Biography:René Descartes|René Descartes]]' ontological distinction between mind and body has been one of the most influential parts of his philosophy.<ref name="Skirry">{{cite web |last1=Skirry |first1=Justin |title=Descartes, Rene: Mind-Body Distinction |url=https://iep.utm.edu/descmind/ |website=Internet Encyclopedia of Philosophy |access-date=19 December 2020}}</ref><ref name="Sandkühler">{{cite book |last1=Sandkühler |first1=Hans Jörg |title=Enzyklopädie Philosophie |date=2010 |publisher=Meiner |url=https://meiner.de/enzyklopadie-philosophie.html |chapter=Ontologie: 2 Zur Begriffs- und Problemgeschichte |access-date=2020-12-16 |archive-date=2021-03-11 |archive-url=https://web.archive.org/web/20210311040207/https://meiner.de/enzyklopadie-philosophie.html |url-status=dead }}</ref> On his view, minds are thinking things while bodies are extended things. ''Thought'' and ''extension'' are two ''attributes'' that each come in various ''modes'' of being. Modes of ''thinking'' include judgments, doubts, volitions, sensations and emotions while the shapes of material things are modes of ''extension''.<ref name="Smith">{{cite web |last1=Smith |first1=Kurt |title=Descartes' Theory of Ideas |url=https://plato.stanford.edu/entries/descartes-ideas/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2018}}</ref> Modes come with a lower degree of reality since they depend for their existence on a substance.<ref>{{cite journal |last1=Nelson |first1=Alan |title=Introduction: Descartes's Ontology |journal=Topoi |date=1997 |volume=16 |issue=2 |pages=103–109 |doi=10.1023/A:1005877628327 |s2cid=170986842 |url=https://philpapers.org/rec/NELIDO-3}}</ref> Substances, on the other hand, can exist on their own.<ref name="Smith"/> Descartes' substance dualism asserts that every finite substance is either a thinking substance or an extended substance.<ref>{{cite journal |last1=Rodriguez-Pereyra |first1=Gonzalo |title=Descartes's Substance Dualism and His Independence Conception of Substance |journal=Journal of the History of Philosophy |date=2008 |volume=46 |issue=1 |pages=69–89 |doi=10.1353/hph.2008.1827 |s2cid=201736234 |url=https://philpapers.org/rec/RODDSD}}</ref><ref>{{cite web |last1=Robinson |first1=Howard |title=Dualism |url=https://plato.stanford.edu/entries/dualism/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref> This position does not entail that minds and bodies ''actually are'' separated from each other, which would defy the intuition that we both have a body and a mind. Instead, it implies that minds and bodies ''can'', at least in principle, be separated, since they are distinct substances and therefore are capable of independent existence.<ref name="Skirry"/><ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Descartes, René}}</ref> A longstanding problem for substance dualism since its inception has been to explain how minds and bodies can [[Philosophy:Problem of mental causation|causally interact]] with each other, as they apparently do, when a volition causes an arm to move or when light falling on the retina causes a visual impression.<ref name="Skirry"/>

[[Biography:Baruch Spinoza|Baruch Spinoza]] is well-known for his ''substance monism:'' the thesis that only one substance exists.<ref name="Dutton">{{cite web |last1=Dutton |first1=Blake D. |title=Spinoza, Benedict De |url=https://iep.utm.edu/spinoza/ |website=Internet Encyclopedia of Philosophy |access-date=21 December 2020}}</ref><ref name="Sandkühler"/> He refers to this substance as "God or Nature", emphasizing both his [[Philosophy:Pantheism|pantheism]] and his [[Philosophy:Naturalism|naturalism]].<ref>{{cite web |last1=Mander |first1=William |title=Pantheism |url=https://plato.stanford.edu/entries/pantheism/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=21 December 2020 |date=2020}}</ref> This substance has an infinite amount of attributes, which he defines as "what the intellect perceives of substance as constituting its essence".<ref>{{cite web |last1=Shein |first1=Noa |title=Spinoza's Theory of Attributes |url=https://plato.stanford.edu/entries/spinoza-attributes/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=21 December 2020 |date=2018}}</ref> Of these attributes, only two are accessible to the human mind: thought and extension. ''Modes'' are properties of a substance that follow from its attributes and therefore have only a dependent form of existence.<ref>{{cite book |last1=Viljanen |first1=Valtteri |title=The Cambridge Companion to Spinoza's Ethics |date=2009 |publisher=Cambridge University Press |url=https://philpapers.org/rec/VILSO |chapter=Spinoza's Ontology|pages=56–78 }}</ref> Spinoza sees everyday-things like rocks, cats or ourselves as mere modes and thereby opposes the traditional [[Biography:Aristotle|Aristotelian]] and Cartesian conception of categorizing them as substances.<ref name="Waller">{{cite web |last1=Waller |first1=Jason |title=Spinoza, Benedict de: Metaphysics |url=https://iep.utm.edu/spinoz-m/ |website=Internet Encyclopedia of Philosophy |access-date=21 December 2020}}</ref> Modes compose deterministic systems in which the different modes are linked to each other as cause and effect.<ref name="Dutton"/> Each deterministic system corresponds to one attribute: one for extended things, one for thinking things, etc. Causal relations only happen within a system while the different systems run in parallel without causally interacting with each other.<ref name="Waller"/> Spinoza calls the system of modes ''Natura naturata'' ("nature natured") and opposes it to ''Natura naturans'' ("nature naturing"), the attributes responsible for the modes.<ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Spinoza, Benedict de (1632–77)}}</ref> Everything in Spinoza's system is necessary: there are no contingent entities. This is so since the attributes are themselves necessary and since the system of modes follows from them.<ref name="Dutton"/>

[[Biography:Christian Wolff (philosopher)|Christian Wolff]] defines ontology as the science of being in general. He sees it as a part of metaphysics besides cosmology, psychology and natural theology.<ref name="Craig">{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Wolff, Christian}}</ref><ref name="Hettche">{{cite web |last1=Hettche |first1=Matt |last2=Dyck |first2=Corey |title=Christian Wolff |url=https://plato.stanford.edu/entries/wolff-christian/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=16 December 2020 |date=2019}}</ref><ref>{{cite book |last1=Wolff |first1=Christian |title=Preliminary Discourse on Philosophy in General |date=1963 |publisher=Indianapolis: Bobbs-Merrill |pages=45–46 |url=https://philpapers.org/rec/WOLPDO-2}}</ref> According to Wolff, it is a deductive science, knowable a priori and based on two fundamental principles: the principle of non-contradiction ("it cannot happen that the same thing is and is not") and the [[Philosophy:Principle of sufficient reason|principle of sufficient reason]] ("nothing exists without a sufficient reason for why it exists rather than does not exist").<ref name="Craig"/><ref name="Sandkühler"/> ''Beings'' are defined by their ''determinations'' or ''predicates'', which cannot involve a contradiction. Determinates come in 3 types: ''essentialia'', ''attributes'', and ''modes''.<ref name="Craig"/> ''Essentialia'' define the nature of a being and are therefore necessary properties of this being. ''Attributes'' are determinations that follow from essentialia and are equally necessary, in contrast to ''modes'', which are merely contingent. Wolff conceives ''existence'' as just one determination among others, which a being may lack.<ref name="Hettche"/> Ontology is interested in being at large, not just in actual being. But all beings, whether actually existing or not, have a sufficient reason.<ref name="Borchert">{{cite book |last1=Borchert |first1=Donald |title=Macmillan Encyclopedia of Philosophy, 2nd Edition |date=2006 |publisher=Macmillan |url=https://philpapers.org/rec/MONMEO-3 |chapter=Ontology, History of}}</ref> The sufficient reason for things without actual existence consists in all the determinations that make up the essential nature of this thing. Wolff refers to this as a "reason of being" and contrasts it with a "reason of becoming", which explains why some things have actual existence.<ref name="Hettche"/>

[[Biography:Arthur Schopenhauer|Arthur Schopenhauer]] was a proponent of [[Philosophy:Voluntarism#Metaphysical voluntarism|metaphysical voluntarism]]:<ref>{{cite web |title=Voluntarism |url=https://www.britannica.com/topic/voluntarism-philosophy |website=Encyclopedia Britannica |access-date=21 November 2020 |language=en}}</ref> he regards will as the underlying and ultimate reality.<ref>{{cite book |last1=Ortegat |first1=P. |last2=Walker |first2=L. J. |title=New Catholic Encyclopedia Volume 14 |page=582 |url=https://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/voluntarism |chapter=Voluntarism}}</ref> Reality as a whole consists only of one will, which is equated with the Kantian [[Philosophy:Thing-in-itself|thing-in-itself]]. Like the Kantian thing-in-itself, the will exists outside space and time. But, unlike the Kantian thing-in-itself, the will has an experiential component to it: it comes in the form of striving, desiring, feeling, etc.<ref>{{cite web |last1=Wicks |first1=Robert |title=Arthur Schopenhauer |url=https://plato.stanford.edu/entries/schopenhauer/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=21 November 2020 |date=2019}}</ref><ref>{{cite book |last1=Young |first1=Julian |title=Schopenhauer |date=2005 |publisher=Routledge |url=https://philpapers.org/rec/YOUS |chapter=3. Metaphysics: The World as Will}}</ref> The manifold of things we encounter in our everyday experiences, like trees or cars, are mere appearances that lack existence independent of the observer. Schopenhauer describes them as objectivations of the will. These objectivations happen in different "steps", which correspond to the platonic forms.<ref>{{cite book |last1=Frauenstädt |first1=Julius |title=Schopenhauer-Lexikon. Ein Philosophisches Wörterbuch, Nach Arthur Schopenhauers Sämmtlichen Schriften Und Handschriftlichem Nachlass |date=1871 |publisher=F. A. Brockhaus |url=http://www.schopenhauers-kosmos.de/Objektivation |chapter=Objektivation}}</ref> All objectivations are grounded in the will. This grounding is governed by the principium individuationis, which enables a manifold of individual things spread out in space and time to be grounded in the one will.<ref name="Kastrup">{{cite book |last1=Kastrup |first1=Bernardo |title=Decoding Schopenhauer's Metaphysics: The Key to Understanding How It Solves the Hard Problem of Consciousness and the Paradoxes of Quantum Mechanics |publisher=John Hunt Publishing |isbn=978-1789044270 |url=https://books.google.com/books?id=-jDtDwAAQBAJ |language=en |chapter=10. Individuality and dissociation|year= 2020 }}</ref>

=== 20th century ===
Dominant approaches to ontology in the ''20th century'' were phenomenology, linguistic analysis and naturalism. ''Phenomenological ontology'', as exemplified by Edmund Husserl and Martin Heidegger, relies for its method on the description of experience. ''Linguistic analysis'' assigns to language a central role for ontology, as seen, for example, in Rudolf Carnap's thesis that the truth value of existence-claims depends on the linguistic framework in which they are made. ''Naturalism'' gives a prominent position to the natural sciences for the purpose of finding and evaluating ontological claims. This position is exemplified by Quine's method of ontology, which involves analyzing the ontological commitments of scientific theories.<ref name="Sandkühler"/><ref name="Dahlstrom"/>

[[Biography:Edmund Husserl|Edmund Husserl]] sees ontology as a ''science of essences''.<ref name="Dahlstrom"/> ''Sciences of essences'' are contrasted with ''factual sciences'': the former are knowable a priori and provide the foundation for the later, which are knowable a posteriori.<ref name="Sandkühler"/><ref name="Gander"/> Ontology as a science of essences is not interested in ''actual facts'', but in the essences themselves, whether they ''have instances or not''.<ref>{{cite book |last1=Føllesdal |first1=Dagfinn |title=A Companion to Phenomenology and Existentialism |date=2006 |publisher=John Wiley & Sons, Ltd |isbn=978-0470996508 |pages=105–114 |url=https://onlinelibrary.wiley.com/doi/10.1002/9780470996508.ch8 |language=en |chapter=Husserl's Reductions and the Role They Play in His Phenomenology|doi=10.1002/9780470996508.ch8 }}</ref> Husserl distinguishes between ''formal ontology'', which investigates the essence of ''objectivity in general'',<ref>{{cite book |last1=Drummond |first1=John J. |title=Historical Dictionary of Husserl's Philosophy |date=2009 |publisher=Scarecrow Press |url=https://philpapers.org/rec/DRUHDO |chapter=Formal ontology}}</ref> and ''regional ontologies'', which study ''regional essences'' that are shared by all entities belonging to the region.<ref name="Dahlstrom">{{cite book |last1=Dahlstrom |first1=D. O. |title=New Catholic Encyclopedia |date=2004 |publisher=Gale |url=https://www.encyclopedia.com/philosophy-and-religion/philosophy/philosophy-terms-and-concepts/ontology |chapter=Ontology}}</ref> Regions correspond to the highest genera of [[Philosophy:Abstract and concrete|concrete entities]]: material nature, personal consciousness and interpersonal spirit.<ref>{{cite journal |last1=Poli |first1=Roberto |title=Husserl's Conception of Formal Ontology |journal=History and Philosophy of Logic |date=1993 |volume=14 |pages=1–14|doi=10.1080/01445349308837207 }}</ref><ref>{{cite book |last1=Moran |first1=Dermot |last2=Cohen |first2=Joseph |title=The Husserl Dictionary |date=2012 |publisher=Continuum |url=https://philpapers.org/rec/MORTHD |chapter=Regional ontology}}</ref> Husserl's method for studying ontology and sciences of essence in general is called eidetic variation.<ref name="Gander">{{cite book |last1=Gander |first1=Hans-Helmuth |title=Husserl Lexikon |publisher=Wissenschaftliche Buchgesellschaft |url=https://philpapers.org/rec/GANHL |chapter=Ontologie|year=2009 }}</ref> It involves imagining an object of the kind under investigation and varying its features.<ref>{{cite book |last1=Drummond |first1=John J. |title=Historical Dictionary of Husserl's Philosophy |date=2009 |publisher=Scarecrow Press |url=https://philpapers.org/rec/DRUHDO |chapter=Eidetic variation}}</ref> The changed feature is ''inessential'' to this kind if the object can survive its change, otherwise it belongs to the ''kind's essence''. For example, a triangle remains a triangle if one of its sides is extended but it ceases to be a triangle if a fourth side is added. Regional ontology involves applying this method to the essences corresponding to the highest genera.<ref>{{cite web |last1=Spear |first1=Andrew D. |title=Husserl, Edmund: Intentionality and Intentional Content |url=https://iep.utm.edu/huss-int/ |website=Internet Encyclopedia of Philosophy |access-date=22 December 2020}}</ref>

Central to [[Biography:Martin Heidegger|Martin Heidegger]]'s philosophy is the notion of ''ontological difference'': the difference between ''being'' as such and specific entities.<ref name="Wheeler">{{cite web |last1=Wheeler |first1=Michael |title=Martin Heidegger |url=https://plato.stanford.edu/entries/heidegger/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |date=2020}}</ref><ref>{{cite book |last1=Schalow |first1=Frank |title=Historical Dictionary of Heidegger's Philosophy |date=2010 |publisher=Scarecrow Press |url=https://philpapers.org/rec/SCHHDO-2 |chapter=Ontological difference}}</ref> He accuses the philosophical tradition of being ''forgetful'' of this distinction, which has led to the mistake of understanding ''being'' as such as a kind of ultimate entity, for example as "idea, energeia, substance, monad or will to power".<ref name="Wheeler"/><ref name="Dahlstrom"/><ref name="Korab-Karpowicz">{{cite web |last1=Korab-Karpowicz |first1=W. J. |title=Heidegger, Martin |url=https://iep.utm.edu/heidegge/ |website=Internet Encyclopedia of Philosophy}}</ref> Heidegger tries to rectify this mistake in his own "fundamental ontology" by focusing on the ''meaning of being'' instead, a project which is akin to contemporary [[Philosophy:Meta-ontology|meta-ontology]].<ref>{{cite book |last1=Inwood |first1=Michael |title=A Heidegger Dictionary |date=1999 |publisher=Wiley-Blackwell |url=https://philpapers.org/rec/INWAHD-3 |chapter=Ontology and fundamental ontology}}</ref><ref>{{cite journal |last1=Inwagen |first1=Peter Van |title=Meta-Ontology |journal=Erkenntnis |date=1998 |volume=48 |issue=2–3 |pages=233–250 |doi=10.1023/A:1005323618026 |url=https://philpapers.org/rec/VANM-3}}</ref> One method to achieve this is by studying the human being, or [[Philosophy:Dasein|Dasein]], in Heidegger's terminology.<ref name="Sandkühler"/> The reason for this is that we already have a ''pre-ontological understanding'' of ''being'' that shapes how we experience the world. [[Philosophy:Phenomenology|Phenomenology]] can be used to make this implicit understanding explicit, but it has to be accompanied by [[Philosophy:Hermeneutics#Heidegger (1889–1976)|hermeneutics]] in order to avoid the distortions due to the ''forgetfulness of being''.<ref name="Wheeler" /> In his later philosophy, Heidegger attempted to reconstruct the "history of being" in order to show how the different epochs in the history of philosophy were dominated by different conceptions of ''being''.<ref>{{cite book |last1=Inwood |first1=Michael |title=A Heidegger Dictionary |date=1999 |publisher=Wiley-Blackwell |url=https://philpapers.org/rec/INWAHD-3 |chapter=History of being}}</ref> His goal is to retrieve the ''original experience of being'' present in the early Greek thought that was covered up by later philosophers.<ref name="Korab-Karpowicz"/>

[[Biography:Nicolai Hartmann|Nicolai Hartmann]] is a 20th-century philosopher within the [[Philosophy:Continental philosophy|continental tradition of philosophy]]. He interprets ontology as [[Biography:Aristotle|Aristotle]]'s science of being qua being: the science of the most general characteristics of entities, usually referred to as categories, and the relations between them.<ref>{{cite book |last1=Hartmann |first1=Nicolai |title=Zur Grundlegung der Ontologie |date=1935 |publisher=W. De Gruyter |url=https://philpapers.org/rec/HARZGD-4 |chapter=1. Kapitel. Die ontologische Grundfrage}}</ref><ref>{{cite book |last1=Aristotle |last2=Reeve |first2=C. D. C. |title=Metaphysics |publisher=Hackett Publishing |isbn=978-1624664410 |url=https://books.google.com/books?id=prGgCwAAQBAJ |language=en |chapter=Book Epsilon|year=2016 }}</ref><ref>{{cite book |last1=Spiegelberg |first1=Herbert |title=The Phenomenological Movement a Historical Introduction |date=1963 |publisher=M. Nijhoff |pages=309–310 |edition=3rd |url=https://philpapers.org/rec/SPITPM-6}}</ref> According to Hartmann, the most general categories are ''moments of being'' (existence and essence), ''modes of being'' (reality and ideality) and ''modalities of being'' (possibility, actuality and necessity). Every entity has both ''existence'' and ''essence''.<ref>{{cite book |last1=Hartmann |first1=Nicolai |title=Zur Grundlegung der Ontologie |date=1935 |publisher=W. De Gruyter |url=https://philpapers.org/rec/HARZGD-4 |chapter=12. Kapitel. Die Trennung von Dasein und Sosein}}</ref> ''Reality'' and ''ideality'', by contrast, are two disjunctive categories: every entity is either real or ideal. Ideal entities are universal, returnable and always existing while real entities are individual, unique and destructible.<ref name="Cicovacki">{{cite book |last1=Cicovacki |first1=Predrag |title=The Analysis of Wonder: An Introduction to the Philosophy of Nicolai Hartmann |date=2014 |publisher=Bloomsbury Publishing US |isbn=978-1623569747 |url=https://books.google.com/books?id=IXnHAgAAQBAJ |language=en |chapter=I.3 Modifications of Being}}</ref> Among the ideal entities are mathematical objects and values.<ref>{{cite book |last1=Mohanty |first1=J. N. |title=Phenomenology. Between Essentialism and Transcendental Philosophy |date=1997 |publisher=Northwestern University Press |chapter=Chapter 3: Nicolai Hartmann's Phenomenological Ontology}}</ref> The ''modalities of being'' are divided into the absolute modalities (actuality and non-actuality) and the relative modalities (possibility, impossibility and necessity). The relative modalities are ''relative'' in the sense that they depend on the absolute modalities: something is possible, impossible or necessary because something else is actual. Hartmann asserts that reality is made up of four [[Integrative level|levels]] (''inanimate'', ''biological'', ''psychological'' and ''spiritual'') that form a hierarchy.<ref name="Poli"/><ref name="Hartmann"/>

[[Biography:Rudolf Carnap|Rudolf Carnap]] proposed that the truth value of ontological statements about the existence of entities depends on the linguistic framework in which these statements are made: they are internal to the framework.<ref name="Hofweber"/><ref name="Dahlstrom"/> As such, they are often trivial in that it just depends on the rules and definitions within this framework. For example, it follows analytically from the rules and definitions within the mathematical framework that numbers exist.<ref name="Leitgeb"/> The problem Carnap saw with traditional ontologists is that they try to make framework-independent or ''external'' statements about what ''really'' is the case.<ref name="Sandkühler"/><ref>{{cite web |last1=Leitgeb |first1=Hannes |last2=Carus |first2=André |title=Rudolf Carnap > H. Tolerance, Metaphysics, and Meta-Ontology (Stanford Encyclopedia of Philosophy |url=https://plato.stanford.edu/entries/carnap/tolerance-metaphysics.html |website=The Stanford Encyclopedia of Philosophy}}</ref> Such statements are at best ''pragmatic considerations'' about which framework to choose and at worst outright ''meaningless'', according to Carnap.<ref>{{cite web |last1=Murzi |first1=Mauro |title=Carnap, Rudolf |url=https://iep.utm.edu/carnap/ |website=Internet Encyclopedia of Philosophy |access-date=23 December 2020}}</ref> For example, there is no matter of fact as to whether realism or [[Philosophy:Idealism|idealism]] is true, their truth depends on the adopted framework.<ref>{{cite book |last1=Blatti |first1=Stephan |last2=Lapointe |first2=Sandra |title=Ontology After Carnap |publisher=Oxford University Press UK |url=https://philpapers.org/rec/SANOAC-5 |chapter=Introduction|year=2016 }}</ref> The job of philosophers is not to discover which things exist by themselves but "conceptual engineering": to create interesting frameworks and to explore the consequences of adopting them.<ref name="Leitgeb">{{cite web |last1=Leitgeb |first1=Hannes |last2=Carus |first2=André |title=Rudolf Carnap |url=https://plato.stanford.edu/entries/carnap/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=23 December 2020 |date=2020}}</ref><ref name="Hofweber">{{cite web |last1=Hofweber |first1=Thomas |title=Logic and Ontology |url=https://plato.stanford.edu/entries/logic-ontology/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=23 December 2020 |date=2020}}</ref> The choice of framework is guided by practical considerations like expedience or fruitfulness since there is no framework-independent notion of truth.<ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Carnap, Rudolf}}</ref>

The notion of [[Philosophy:Ontological commitment|ontological commitment]] plays a central role in [[Biography:Willard Van Orman Quine|Willard Van Orman Quine]]'s contributions to ontology.<ref>{{cite book |last1=Craig |first1=Edward |title=Routledge Encyclopedia of Philosophy |date=1996 |publisher=Routledge |url=https://philpapers.org/rec/BEAREO |chapter=Ontological commitment}}</ref><ref>{{cite web |title=Ontology |url=https://www.britannica.com/topic/ontology-metaphysics |website=Encyclopedia Britannica |access-date=13 December 2020 |language=en}}</ref> A theory is ontologically committed to an entity if that entity must exist in order for the theory to be true.<ref name="Bricker">{{cite web |last1=Bricker |first1=Phillip |title=Ontological Commitment |url=https://plato.stanford.edu/entries/ontological-commitment/ |website=The Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |access-date=13 December 2020 |date=2016}}</ref> Quine proposed that the best way to determine this is by translating the theory in question into first-order predicate logic. Of special interest in this translation are the logical constants known as [[Existential quantification|existential quantifiers]], whose meaning corresponds to expressions like "there exists..." or "for some...". They are used to [[First-order logic#Free and bound variables|bind the variables]] in the expression following the quantifier.<ref>{{cite book |last1=Magnus |first1=P. D. |last2=Ichikawa |first2=Jonathan Jenkins |title=Forall X |date=2020 |publisher=Creative Commons: Attribution-ShareAlike 3.0 |url=https://philpapers.org/rec/MAGFXU |chapter=V. First-order logic|edition=UBC }}</ref> The ontological commitments of the theory then correspond to the variables bound by existential quantifiers.<ref>{{cite book |last1=Schaffer |first1=Jonathan |title=Metametaphysics: New Essays on the Foundations of Ontology |publisher=Oxford University Press |pages=347–383 |url=https://philpapers.org/rec/SCHOWG |chapter=On What Grounds What|year=2009 }}</ref> This approach is summed up by Quine's famous dictum that "[t]o be is to be the value of a variable".<ref name="Quine">{{cite journal |last1=Quine |first1=Willard Van Orman |title=On What There Is |journal=Review of Metaphysics |date=1948 |volume=2 |issue=5 |pages=21–38 |url=https://philpapers.org/rec/QUIOWT-7}}</ref> This method by itself is not sufficient for ontology since it depends on a theory in order to result in ontological commitments. Quine proposed that we should base our ontology on our best scientific theory.<ref name="Bricker"/> Various followers of Quine's method chose to apply it to different fields, for example to "everyday conceptions expressed in natural language".<ref>{{cite book |last1=Inwagen |first1=Peter van |title=Oxford Studies in Metaphysics, Volume 1 |date=2004 |publisher=Clarendon Press |pages=107–138 |url=https://philpapers.org/rec/VANATO-2 |chapter=A Theory of Properties}}</ref><ref>{{cite book |last1=Kapelner |first1=Zsolt-kristof |title=Reconciling Quinean and neo-Aristotelian Metaontology |date=2015 |url=http://www.etd.ceu.hu/2015/kapelner_zsolt-kristof.pdf |chapter=3. Quinean Metaontology}}</ref>

== Other ontological topics ==
=== Ontological formations ===
The concept of ''ontological formations'' refers to formations of social relations understood as dominant ways of living. Temporal, spatial, corporeal, [[Philosophy:Epistemology|epistemological]], and performative relations are taken to be central to understanding a dominant formation. That is, a particular ontological formation is based on how ontological categories of time, space, embodiment, knowing and performing are lived—objectively and subjectively. Different ontological formations include the ''customary'' (including the tribal), the ''traditional'', the ''modern'', and the ''postmodern''. The concept was first introduced by [[Biography:Paul James (academic)|Paul James]] in 2006, together with a series of writers including Damian Grenfell and Manfred Steger.<ref>{{Cite book |last=James |first=Paul |url=https://www.academia.edu/1642214 |title=Globalism, Nationalism, Tribalism: Bringing Theory Back. In Volume 2 of Towards a Theory of Abstract Community |publisher=Sage Publications |year=2006 |location=London, England |language=en-uk |}}</ref>

In the ''[[Philosophy:Engaged theory|engaged theory]]'' approach, ontological formations are seen as layered and intersecting rather than singular formations. They are 'formations of being'. This approach avoids the usual problems of a Great Divide being posited between the modern and the pre-modern. From a philosophical distinction concerning different formations of being, the concept then provides a way of translating into practical understandings concerning how humans might design cities and communities that live creatively across different ontological formations, for example cities that are not completely dominated by modern valences of spatial configuration. Here the work of Tony Fry is important.<ref>{{Cite book |last1=James |first1=Paul |title=Design in the Borderlands |publisher=Routledge |year=2014 |editor=Fry |editor-first=Tony |location=London, England |language=en-uk |chapter=Urban Design in the Global South: Ontological Design in Practice |editor2=Kalantidou |editor-first2=Eleni |chapter-url=https://www.academia.edu/9161853}}</ref>

=== Ontology of fictional characters ===
According to [[Biography:Edward N. Zalta|Edward N. Zalta]], the ontology of fiction analyses such sentences as:<ref>[[Biography:Edward N. Zalta|Zalta, Edward N.]] 2009. "Fictional truth, objects, and characters." pp. 267–269 in ''A Companion to Metaphysics'' (2nd ed.), edited by J. Kim G. S. Rosenkrantz, and E. Sosa. Chichester, UK: [[Company:Wiley-Blackwell|Wiley–Blackwell]]. {{ISBN|978-1405152983}}. p. 267.</ref>
* '[[Biography:Nero|Nero]] worshipped (the god) [[Religion:Mars (mythology)|Mars]];'
* '[[Religion:Mars (mythology)|Mars]], the god, does not exist;' and
* 'Eliza Doolittle, in [[Biography:George Bernard Shaw|George Bernard Shaw]]'s ''Pygmalion'', is a flower girl.'

According to Amie L. Thomasson, fictional discourse can be of four sorts:
* Uttered ''within'' works of fiction;
* Philosophical exercises such as '[[Religion:Captain Marvel (DC Comics)|Captain Marvel]] does not exist';
* Treating fictional characters as if they were 'real', such as 'Superman can leap tall buildings;' and
* Discourse ''about'' works of fiction, such as 'Professor Higgins was created by [[Biography:George Bernard Shaw|George Bernard Shaw]]'.<ref name="Thomasson">{{cite encyclopedia |title=Fictional Entities |last=Thomasson |first=Amie L. |url=https://uh.edu/~garson/Thomasson%20-%20Fictional%20Entities.pdf |encyclopedia=A Companion to Metaphysics |editor-last1=Kim |editor-first1=Jaegwon |editor-last2=Sosa |editor-first2=Ernest |editor-last3=Rosenkrantz |editor-first3=Gary S. |publisher=Wiley-Blackwell |year=2009 |edition=Second}}</ref>

[[Biography:Jeremy Bentham|Jeremy Bentham]] distinguished three kinds of entities:<ref>Harrison, R. (2009). Jeremy Bentham, p. 145 in ''A Companion to Metaphysics'', ed. Kim, J., Rosenkrantz, G.S., Sosa, E., Wiley–Blackwell, Chichester UK, 2nd ed., {{ISBN|978-1405152983}}.</ref>
* the ''real'': those that can be perceived, or can be inferred from perception.
* the ''fictitious'': abstractions that referred to perceptible things.
* the ''fabulous'': those that can be found only in the imagination, where the word 'exist' applies to such only in the sense that they do not really exist.

Francis Herbert Bradley thought that real things exist respectively at particular times and places. He recognised several kinds of entity:<ref>Stock, G. (2009). Francis Herbert Bradley, pp. 155–158 in ''A Companion to Metaphysics'', ed. Kim, J., Rosenkrantz, G.S., Sosa, E., Wiley–Blackwell, Chichester UK, 2nd ed., {{ISBN|978-1405152983}}, p. 157.</ref>
* the genuinely historical;
* the fictional;
* the real;
* the merely imagined;
* the existent; and
* the non-existent.

[[Biography:Alexius Meinong|Alexius Meinong]] would put fictional entities into the category which he called ''subsistence''.<ref name=":0" /> This category contains objects that neither exist spatially or non-spatially. However, they do have properties. The properties are given to these objects in the way they are said to be described. For example, we can talk about the tall unicorn even though the tall unicorn does not exist. We can say the unicorn is in fact tall because this follows from the properties in which the object is characterized.<ref name=":0">{{cite encyclopedia |title=Fictional Entities |last1=Kroon |first1=Fred |last2=Voltolini |first2=Alberto |editor-last=Zalta |editor-first=Edward N. |encyclopedia=The Stanford Encyclopedia of Philosophy |year=2018 |url=https://plato.stanford.edu/archives/win2018/entries/fictional-entities }}</ref>

=== Ontological and epistemological certainty ===
[[Biography:René Descartes|René Descartes]], with ''[[Philosophy:Cogito, ergo sum|cogito, ergo sum]]'' (''je pense donc je suis,'' "I think, therefore I am"), argued that a person's thinking agency, his ''res cogitans'', as distinct from his material body, his ''res extensa'', is something that we can know exists with [[Philosophy:Epistemology|epistemological]] certainty. Descartes argued further that this knowledge could lead to a proof of the certainty of the [[Philosophy:Existence of God|existence of God]], using the [[Philosophy:Ontological argument|ontological argument]] that had been formulated first by [[Biography:Anselm of Canterbury|Anselm of Canterbury]].<ref>{{Cite web|url=https://www.iep.utm.edu/ont-arg/|title=Anselm: Ontological Argument for God's Existence.|website=IEP}}</ref>

=== Body and environment, questioning the meaning of being ===
Schools of subjectivism, objectivism and [[Relativism|relativism]] existed at various times in the 20th century, and the [[Philosophy:Postmodernism|postmodernists]] and body philosophers tried to reframe all these questions in terms of bodies taking some specific action in an environment. This relied to a great degree on insights derived from scientific research into animals taking instinctive action in natural and artificial settings—as studied by [[Biology:Biology|biology]], [[Earth:Ecology|ecology]],<ref>{{Cite web |last=Smith |first=Barry |date=2001 |title=Objects and their environments: From Aristotle to ecological ontology |url=https://ontology.buffalo.edu/smith/articles/napflion.pdf |access-date=2023-11-26 |language=en}}</ref> and [[Cognitive science|cognitive science]].{{Citation needed|date=January 2012}}

The processes by which bodies related to environments became of great concern, and the idea of [[Philosophy:Being|being]] itself became difficult to really define. What did people mean when they said "A is B", "A must be B", "A was B"...? Some linguists advocated dropping the verb "to be" from the English language, leaving "E&nbsp;Prime", supposedly less prone to bad abstractions. Others, mostly philosophers, tried to dig into the word and its usage. [[Biography:Martin Heidegger|Martin Heidegger]] distinguished ''human being'' as ''existence'' from the being of things in the world. Heidegger proposes that our way of being human and the way the world is for us are cast historically through a fundamental ontological questioning. These fundamental ontological categories provide the basis for communication in an age: a horizon of unspoken and seemingly unquestionable background meanings, such as human beings understood unquestioningly as subjects and other entities understood unquestioningly as objects. Because these basic ontological meanings both generate and are regenerated in everyday interactions, the locus of our way of being in a historical epoch is the communicative event of language in use.<ref name="Hyde">Hyde, R. Bruce. "Listening Authentically: A Heideggerian Perspective on Interpersonal Communication". In ''Interpretive Approaches to Interpersonal Communication'', edited by Kathryn Carter and Mick Presnell. State University of New York Press, 1994. {{ISBN|0791418472}}.</ref> For Heidegger, however, communication in the ''first'' place is not among human beings, but language itself shapes up in response to questioning (the inexhaustible meaning of) being.<ref>Heidegger, Martin. 1971 [1959]. ''On the Way to Language.'' New York: Harper & Row.

''original'': 1959. ''Unterwegs zur Sprache'' Neske. Pfullingen.</ref> Even the focus of traditional ontology on the 'whatness' or ''quidditas'' of beings in their substantial, standing presence can be shifted to pose the question of the 'whoness' of human being itself.<ref>{{Cite book |last=Eldred |first=Michael |title=Social ontology: recasting political philosophy through a phenomenology of whoness |date=2008 |publisher=Ontos Verl |isbn=978-3-938793-78-7 |location=Frankfurt |pages=xiv, 688 |url=http://www.arte-fact.org/sclontlg.html}}</ref>

=== Ontology and language ===
Some philosophers suggest that the question of "What is?" is (at least in part) an issue of ''usage'' rather than a question about facts.<ref>{{cite book | title= Introduction to an Ontology of Intellectual Property | publisher=The Scitech Lawyer, ABA| author=Carvalko, Joseph | date= Summer 2005 }}</ref> This perspective is conveyed by an analogy made by [[Biography:Donald Davidson (philosopher)|Donald Davidson]]: Suppose a person refers to a 'cup' as a 'chair' and makes some comments pertinent to a cup, but uses the word 'chair' consistently throughout instead of 'cup'. One might readily catch on that this person simply calls a 'cup' a 'chair' and the oddity is explained.<ref name="Davidson">{{cite journal|author=Davidson, Donald|year=1974|title=On the very idea of a conceptual scheme|url=http://files.meetup.com/328570/davidson_on-the-very-idea.pdf|journal=Proceedings and Address of the American Philosophical Association|volume=47|pages=5–20}}

Davidson refers to a 'ketch' and a 'yawl' (p. 18).</ref> Analogously, if we find people asserting 'there are' such-and-such, and we do not ourselves think that 'such-and-such' exist, we might conclude that these people are not nuts (Davidson calls this assumption 'charity'), they simply use 'there are' differently than we do. The question of ''What is?'' is at least partly a topic in the philosophy of language, and is not entirely about ontology itself.<ref name="Krieger">{{cite journal|author=Kriegel, Uriah|year=2011|title=Two defenses of common-sense ontology|url=http://www.uriahkriegel.com/downloads/defenses.pdf|url-status=dead|journal=Dialectica|volume=65|issue=2|pages=177–204|doi=10.1111/j.1746-8361.2011.01262.x|archive-url=https://web.archive.org/web/20181221041700/http://uriahkriegel.com/downloads/defenses.pdf|archive-date=2018-12-21|access-date=2013-04-27}}</ref> This viewpoint has been expressed by [[Biography:Eli Hirsch|Eli Hirsch]].<ref name="Hirsch">Hirsch, Eli. 2011. "[https://books.google.com/books?id=iPRqtcjeHPsC&pg=PA144 Physical-object ontology, verbal disputes and common sense]." pp. 144–177 in ''Quantifier Variance and Realism: Essays in Metaontology''. New York: [[Organization:Oxford University Press|Oxford University Press]]. {{ISBN|978-0199732111}}.

First published as "[http://onlinelibrary.wiley.com/doi/10.1111/j.1933-1592.2005.tb00506.x/abstract Physical-Object Ontology, Verbal Disputes, and Common Sense]."</ref><ref name="Hirsch1">Hirsch, Eli. 2011. "[https://books.google.com/books?id=iPRqtcjeHPsC&pg=PA68 Quantifier variance and realism]." pp. 68–95 in ''Quantifier Variance and Realism: Essays in Metaontology''. New York: Oxford University Press. {{ISBN|978-0199732111}}.

First published as "[http://onlinelibrary.wiley.com/doi/10.1111/j.1758-2237.2002.tb00061.x/abstract Quantifier variance and realism]."</ref>

Hirsch interprets [[Biography:Hilary Putnam|Hilary Putnam]] as asserting that different concepts of "the existence of something" can be correct.<ref name=Hirsch1 /> This position does not contradict the view that some things do exist, but points out that different 'languages' will have different rules about assigning this property.<ref name=Hirsch1 /><ref name="Hirsch2">{{cite book|author=Hirsch, Eli|title=Ernest Sosa and His Critics|publisher=Blackwell|year=2004|isbn=978-0470755471|editor=John Greco|pages=224–232|chapter=Sosa's Existential Relativism|chapter-url=https://books.google.com/books?id=Moz8OeX4w8AC&pg=PA224}}</ref> How to determine the 'fitness' of a 'language' to the world then becomes a subject for investigation.

Common to all Indo-European copula languages is the double use of the verb "to be" in both stating that entity X exists ("X is.") as well as stating that X has a property ("X is P"). It is sometimes argued that a third use is also distinct, stating that X is a member of a class ("X is a C"). In other language families these roles may have completely different verbs and are less likely to be confused with one another. For example they might say something like "the car has redness" rather than "the car is red". Hence any discussion of "being" in Indo-European language philosophy may need to make distinctions between these senses.{{citation needed|date=September 2017}}

=== Ontology and human geography ===
In human geography there are two types of ontology: small "o" which accounts for the practical orientation, describing functions of being a part of the group, thought to oversimplify and ignore key activities. The other "o", or big "O", systematically, logically, and rationally describes the essential characteristics and universal traits. This concept relates closely to Plato's view that the human mind can only perceive a bigger world if they continue to live within the confines of their "caves". However, in spite of the differences, ontology relies on the symbolic agreements among members. That said, ontology is crucial for the axiomatic language frameworks.<ref>Harvey, F. 2006. "Ontology. pp. 341–343 in ''Encyclopedia of Human Geography'', edited by B. Warf. Thousand Oaks, CA: SAGE Publications.</ref>

=== Anthropology ===
The topic of ontology has received increased attention in [[Social:Anthropology|anthropology]] since the 1990s. This is sometimes termed the "[[Philosophy:Ontological turn|ontological turn]]".<ref name="Scott2013">{{cite journal |last1=Scott |first1=Michael W. |title=The anthropology of ontology (religious science?) |date=2013 |issue=4 |journal=The Journal of the Royal Anthropological Institute |volume=19 |pages=859–872 |doi=10.1111/1467-9655.12067 |jstor=42001687 |url=https://www.jstor.org/stable/42001687 |quote=Since roughly the 1990s, a growing number of anthropologists have become interested in the study of ontology – the investigation and theorization of diverse experiences and understandings of the nature of being itself. This generally takes the form of ethnographic accounts of indigenous non-Western modes and models of being, presented in more or less explicit contrast with aspects of a Euro-American or modern ontology imputed to conventional anthropology.}}</ref> This type of inquiry is focused on how people from different cultures experience and understand the nature of being. Specific interest in this regard has been given to the ontological outlook of indigenous people and how their outlook tends to differ from a more Western perspective.<ref name="Scott2013"/><ref name="Heywood2012">{{cite journal |last1=Heywood |first1=Paolo |title=Anthropology and What There Is: Reflections on 'Ontology' |journal=The Cambridge Journal of Anthropology |date=2012 |volume=30 |issue=1 |pages=143–151 |doi=10.3167/ca.2012.300112 |jstor=43610895 |url=https://www.jstor.org/stable/43610895 |issn=0305-7674}}</ref> As an example of this contrast, it has been argued that various indigenous communities ascribe [[Philosophy:Intentionality|intentionality]] to non-human entities, like plants, forests, or rivers. This outlook is known as [[Philosophy:Animism|animism]].<ref>{{cite journal |last1=Ludwig |first1=David |last2=Weiskopf |first2=Daniel A. |title=Ethnoontology: Ways of world-building across cultures |journal=Philosophy Compass |date=September 2019 |volume=14 |issue=9 |doi=10.1111/phc3.12621 |s2cid=199516840 |quote= Consider the animism debate. Animists consider nonhuman entities (e.g., plants, forests, or rivers) as intentional actors (Harvey, 2005). There is substantial evidence that animism is a widespread metaphysical view. For example, the Nayaka people of South India consider not only certain animals but also stones, hills, cups, and knives to be devaru: beings that stand in active, quasi-social relationships with them (Bird-David, 1999). Devaru are aspects of a larger kin structure that incorporates potential "partners" in the nonhuman world. In addition to these ethnographic observations, there are intriguing cross-cultural similarities in animist ontologies. Indigenous communities around the world tend to be much more permissive in their ascription of intentionality than Western participants (Ojalehto, Douglas, & García, 2017).|doi-access=free }}</ref>

=== Reality and actuality ===
According to [[Biography:Alfred North Whitehead|Alfred N. Whitehead]], for ontology, it is useful to distinguish the terms 'reality' and 'actuality'. In this view, an 'actual entity' has a philosophical status of fundamental ontological priority, while a 'real entity' is one which may be actual, or may derive its reality from its logical relation to some actual entity or entities. For example, an occasion in the life of Socrates is an actual entity. But Socrates' being a man does not make 'man' an actual entity, because it refers indeterminately to many actual entities, such as several occasions in the life of Socrates, and also to several occasions in the lives of [[Biography:Alcibiades|Alcibiades]], and of others. But the notion of man is real; it derives its reality from its reference to those many actual occasions, each of which is an actual entity. An actual occasion is a concrete entity, while terms such as 'man' are abstractions from many concrete relevant entities.{{Citation needed|date=May 2020}}

According to Whitehead, an actual entity must earn its philosophical status of fundamental ontological priority by satisfying several philosophical criteria, as follows:{{Citation needed|date=May 2020}}
* There is no going behind an actual entity, to find something more fundamental in fact or in efficacy. This criterion is to be regarded as expressing an axiom, or postulated distinguished doctrine.
* An actual entity must be completely determinate in the sense that there may be no confusion about its identity that would allow it to be confounded with another actual entity. In this sense an actual entity is completely concrete, with no potential to be something other than itself. It is what it is. It is a source of potentiality for the creation of other actual entities, of which it may be said to be a part cause. Likewise it is the concretion or realization of potentialities of other actual entities which are its partial causes.
* Causation between actual entities is essential to their actuality. Consequently, for Whitehead, each actual entity has its distinct and definite extension in physical [[Physics:Minkowski space|Minkowski space]], and so is uniquely identifiable. A description in Minkowski space supports descriptions in time and space for particular observers.
* It is part of the aim of the philosophy of such an ontology as Whitehead's that the actual entities should be all alike, ''qua'' actual entities; they should all satisfy a single definite set of well stated ontological criteria of actuality.

Whitehead proposed that his notion of an occasion of experience satisfies the criteria for its status as the philosophically preferred definition of an actual entity. From a purely logical point of view, each occasion of experience has in full measure the characters of both objective and subjective reality. Subjectivity and objectivity refer to different aspects of an occasion of experience, and in no way do they exclude each other.<ref>[[Biography:Alfred North Whitehead|Whitehead, Alfred N.]] 1929. ''Process and Reality''. Cambridge: {{wipe|Cambridge University Press}}. ''passim''.</ref>

Examples of other philosophical proposals or candidates as actual entities, in this view, are Aristotle's 'substances', Leibniz' monads, and Descartes' ''res verae'', and the more modern 'states of affairs'. Aristotle's substances, such as Socrates, have behind them as more fundamental the 'primary substances', and in this sense do not satisfy Whitehead's criteria. Whitehead is not happy with Leibniz' monads as actual entities because they are "windowless" and do not cause each other. 'States of affairs' are often not closely defined, often without specific mention of extension in physical Minkowski space; they are therefore not necessarily processes of becoming, but may be as their name suggests, simply static states in some sense. States of affairs are contingent on particulars, and therefore have something behind them.<ref>[[Biography:David Malet Armstrong|Armstrong, D. M.]] (1997). ''A World of States of Affairs'', Cambridge University Press, Cambridge, England, {{ISBN|0521580641}}, p. 1.</ref> One summary of the Whiteheadian actual entity is that it is a process of becoming. Another summary, referring to its causal linkage to other actual entities, is that it is "all window", in contrast with Leibniz' windowless monads.{{Citation needed|date=May 2020}}

This view allows philosophical entities other than actual entities to really exist, but not as fundamentally and primarily factual or causally efficacious; they have existence as abstractions, with reality only derived from their reference to actual entities. A Whiteheadian actual entity has a unique and completely definite place and time. Whiteheadian abstractions are not so tightly defined in time and place, and in the extreme, some are timeless and placeless, or 'eternal' entities. All abstractions have logical or conceptual rather than efficacious existence; their lack of definite time does not make them unreal if they refer to actual entities. Whitehead calls this 'the ontological principle'.{{Citation needed|date=May 2020}}

=== Microcosmic ontology ===
There is an established and long philosophical history of the concept of atoms as microscopic physical objects. They are far too small to be visible to the naked eye. It was as recent as the nineteenth century that precise estimates of the sizes of putative physical [[Physics:Atom|atom]]s began to become plausible. Almost direct empirical observation of atomic effects was due to the theoretical investigation of [[Brownian motion]] by [[Biography:Albert Einstein|Albert Einstein]] in the very early twentieth century. Even then, the real existence of atoms was debated by some. Such debate might be labeled 'microcosmic ontology'. Here the word 'microcosm' is used to indicate a physical world of small entities, such as for example atoms.<ref>{{Cite web |title=Definition of Microcosm |url=https://www.merriam-webster.com/dictionary/microcosm |access-date=2022-09-13 |website=www.merriam-webster.com |language=en}}</ref>

Subatomic particles are usually considered to be much smaller than atoms. Their real or actual existence may be very difficult to demonstrate empirically.<ref>Kaiser, D. 1994. "Niels Bohr's legacy in contemporary particle physics." pp. 257–268 in ''Niels Bohr and Contemporary Philosophy'', edited by J. Faye and H. J. Folse. Dordrecht, Holland: [[Company:Springer Publishing|Springer]]. {{ISBN|978-9048142996}}. s. 4 ("Questions of ontology and particle physics phenomenology"). pp. 262–264.</ref> A distinction is sometimes drawn between actual and [[Physics:Virtual particle|virtual]] subatomic particles. Reasonably, one may ask, in what sense, if any, do virtual particles exist as physical entities? For atomic and subatomic particles, difficult questions arise, such as do they possess a precise position, or a precise momentum? A question that continues to be controversial is "to what kind of physical thing, if any, does the [[Physics:Quantum mechanics|quantum mechanical]] [[Wave function|wave function]] refer?"<ref name="Isham">Isham, C. J. 1995. ''Lectures on Quantum Theory: Mathematical and Structural Foundations.'' London, England: Imperial College Press. {{ISBN|1860940005}}. pp. 63–67.</ref>

=== Ontological argument ===
{{Main|Philosophy:Ontological argument}}

In the [[Philosophy:Christian philosophy|Western Christian]] tradition, in his 1078 work ''[[Philosophy:Proslogion|Proslogion]]'', [[Biography:Anselm of Canterbury|Anselm of Canterbury]] proposed what is known as 'the ontological argument' for the existence of God.{{NoteTag|"There are three main periods in the history of ontological arguments. The first was in 11th century, when St. Anselm of Canterbury came up with the first ontological argument."<ref>Szatkowski, Miroslaw, ed. 2012. ''Ontological Proofs Today''. Ontos Verlag. p.&nbsp;22.</ref>}} Anselm defined God as "that than which nothing greater can be thought", and argued that this being must exist in the mind, even in the mind of the person who denies the existence of God. He suggested that, if the greatest possible being exists in the mind, it must also exist in reality. If it only exists in the mind, then an even greater being must be possible—one which exists both in the mind and in reality. Therefore, this greatest possible being must exist in reality. Seventeenth century French philosopher [[Biography:René Descartes|René Descartes]] deployed a similar argument. Descartes published several variations of his argument, each of which centred on the idea that God's existence is immediately inferable from a "clear and distinct" idea of a supremely perfect being. In the early eighteenth century, Gottfried Leibniz augmented Descartes's ideas in an attempt to prove that a "supremely perfect" being is a coherent concept. [[Biography:Norman Malcolm|Norman Malcolm]] revived the ontological argument in 1960 when he located a second, stronger ontological argument in Anselm's work; [[Biography:Alvin Plantinga|Alvin Plantinga]] challenged this argument and proposed an alternative, based on [[Modal logic|modal logic]]. Attempts have also been made to validate Anselm's proof using an automated theorem prover.<ref>{{Citation |last1=Benzmuller |first1=Christoph |title=Automating Gödel's Ontological Proof of God's Existence with Higher-order Automated Theorem Provers |date=2014 |url=https://page.mi.fu-berlin.de/cbenzmueller/papers/C40.pdf |work=ECAI 2014 |pages=93–98 |access-date=2023-11-26 |doi=10.3233/978-1-61499-419-0-93 |last2=Woltzenlogel Paleo |first2=Bruno|s2cid=46020663 }}</ref> 

More recently, [[Biography:Kurt Gödel|Kurt Gödel]] proposed a [[Mathematical logic|formal argument]] [[Philosophy:Gödel's ontological proof|for God's existence]]. Other [[Philosophy:Existence of God|arguments for God's existence]] have been advanced, including those made by Islamic philosophers [[Biography:Mulla Sadra|Mulla Sadra]]<ref>{{Cite book |last=Ayatollahy |first=Hamidreza |url=https://www.academia.edu/8187996 |title=Mulla Sadra's Seddiqin Argument for the Existence of God An Islamic Response to Hume and Kant}}</ref> and Allama Tabatabai.<ref>{{Cite web |last=Tabatabai |first=Allama |title=Encyclopaedia Iranica |url=https://iranicaonline.org/ |access-date=2023-11-18 |website=iranicaonline.org |language=en-US}}</ref>

===Hintikka's locution for existence===

[[Biography:Jaakko Hintikka|Jaakko Hintikka]] puts the view that a useful explication of the notion of existence is in the words "one can find", implicitly in some world or [[Philosophy:Domain of discourse|universe of discourse]].<ref>[[Biography:Jaakko Hintikka|Hintikka, Jaakko.]] 1998. ''Paradigms for Language Theory and Other Essays''. Dordrecht, Holland: [[Company:Springer Science+Business Media|Springer Science+Business Media]]. {{ISBN|978-9048149308}}. p. 3.</ref>

== See also ==
{{cols}}
* {{annotated link|Philosophy:Entity}}
* {{annotated link|Philosophy:Hauntology}}
{{colend}}

== Notes ==
{{NoteFoot}}

== References ==
{{Reflist
|refs =
<ref name="Petrov">{{cite book |title = Ontological Landscapes: Recent Thought on Conceptual Interfaces Between Science and Philosophy |chapter=Chapter VI: Process ontology in the context of applied philosophy |first = Vesselin |last = Petrov |editor = Vesselin Petrov |chapter-url = https://books.google.com/books?id=bA_5CzzqkEMC&pg=PA137 |pages=137ff|isbn = 978-3868381078 |year=2011 |publisher = Ontos Verlag }}</ref>
}}

== External links ==
* {{SEP|logic-ontology|Logic and Ontology|Thomas Hofweber|October 11, 2017}}.
* {{cite web|url=https://philpapers.org/archive/AUGTBO-2.pdf|title=The birth of ontology|author=Barry Smith}}

{{metaphysics}}

[[Category:Meaning (philosophy of language)]]

{{Sourceattribution|Ontology}}