Ontology

==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" ([[Count noun|singular or plural]]). This usage contrasts with the meaning of "ontology" ([[Mass noun|only singular]]) as a branch of philosophy: the ''science of being'' in general.<ref>{{cite journal |last=Simons |first=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=Merriam-Webster |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 |last=Paul |first=L. A. |title=Being, Freedom, and Method: Themes From the Philosophy of Peter van Inwagen |date=2017 |publisher=Oxford University Press |chapter-url=https://philpapers.org/rec/PAUAOC-4 |via=PhilPapers |chapter=A One Category Ontology|pages=32–62 |url-status=live |archive-url=https://web.archive.org/web/20240128093746/https://philpapers.org/rec/PAUAOC-4 |archive-date= Jan 28, 2024 }}</ref><ref name="Thomasson2">{{cite web |last=Thomasson |first=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 |url-status=live |archive-url=https://archive.today/20150708032808/http://plato.stanford.edu/entries/categories/ |archive-date= 8 Jul 2015 }}</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 |last=Brassier |first=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 |last=Epstein |first=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=[[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 |last=Quine |first=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 [[Willard Van Orman Quine]], which is why it has been referred to as the [[Meta-ontology#Quinean approach|Quinean]] approach to [[meta-ontology]].<ref name=Schaffer/><ref>{{cite web |last=Bricker |first=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 (philosopher)|E. J. Lowe]]'s four-category-ontology.<ref name=Lowe>{{cite book |last1=Lowe |first1=E. J. |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 |last=Miller |first=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 [[Grounding (metaphysics)|metaphysical grounding]].<ref>{{cite web |last=Tahko |first=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 |last=Mehta |first=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 |last=Schaffer |first=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 [[Continental philosophy]] comes from [[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 |last=Poli |first=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 |last=Hartmann |first=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 |last=Bergmann |first=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 |last=Rosenkrantz |first=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 |last=Esfeld |first=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 [[Ludwig Wittgenstein]]: "The world is the totality of facts, not of things".<ref>{{cite book |last=Wittgenstein |first=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 |last=Armstrong |first=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 |last=Skyrms |first=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>

[[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> [[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 [[homogeneity|homogeneous]], internally indifferentiable "[[wikt:blob|blobs]]".<ref name="Inwagen">{{cite journal |last=Inwagen |first=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 |via=PhilPapers |url-status=live |archive-url= https://web.archive.org/web/20221007145421/https://philpapers.org/rec/VANRVC |archive-date= Oct 7, 2022 }}</ref><ref name=Bradley/><ref>{{cite book |last=Vallicella |first=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>

[[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 |last=Robinson |first=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 [[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 |last=Rodriguez-Pereyra |first=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, [[meta-ontology]].
# ''[[Ontology (information science)|Domain ontology]]'': concepts relevant to a particular topic, [[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 [[biomedical sciences]], ontologies have been used to create terminologies for various aspects of living organism or medical applications. A prominent example is the [[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 [[OBO Foundry|OBO]] (Open Biological Ontologies) project.<ref name=":1"/>