Editing Ontology (section)

=== Other concepts ===
Ontological dependence is a relation between entities. An entity depends ontologically on another entity if the first entity cannot exist without the second entity.<ref>{{multiref | {{harvnb|Ney|2014|pp=53–55}} | {{harvnb|Fine|1995|pp=269–270}} }}</ref> For instance, the surface of an apple cannot exist without the apple.<ref>{{harvnb|Nuñez Erices|2019|pp=15–16}}</ref> An entity is ontologically independent if it does not depend on anything else, meaning that it is fundamental and can exist on its own. Ontological dependence plays a central role in ontology and its attempt to describe reality on its most fundamental level.<ref>{{multiref | {{harvnb|Ney|2014|pp=53–55}} | {{harvnb|Fine|1995|pp=269–270}} }}</ref> It is closely related to [[Grounding (metaphysics)|metaphysical grounding]], which is the relation between a ground and the facts it explains.<ref>{{multiref | {{harvnb|Tahko|Lowe|2020|loc=§ 5. Ontological Dependence and Metaphysical Grounding}} | {{harvnb|Ney|2014|pp=53–55}} }}</ref>

[[File:Willard Van Orman Quine on Bluenose II in Halifax NS harbor 1980.jpg|thumb|upright=.8|alt=Photo of Willard Van Orman Quine|[[Willard Van Orman Quine]] used the concept of [[ontological commitment]]s to analyze theories.]]

An [[ontological commitment]] of a person or a theory is an entity that exists according to them.<ref>{{multiref | {{harvnb|Ney|2014|p=31}} | {{harvnb|Jubien|1998|loc=Lead Section}} }}</ref> For instance, a person who [[Theism|believes in God]] has an ontological commitment to [[God]].<ref>{{harvnb|Jubien|1998|loc=Lead Section}}</ref> Ontological commitments can be used to analyze which ontologies people explicitly defend or implicitly assume. They play a central role in contemporary metaphysics when trying to decide between competing theories. For example, the [[Quine–Putnam indispensability argument]] defends [[mathematical Platonism]], asserting that numbers exist because the best scientific theories are ontologically committed to numbers.<ref>{{multiref | {{harvnb|Colyvan|2001|p=23}} | {{harvnb|Bangu|2012|pp=26–27}} | {{harvnb|Ney|2014|pp=40–43}} | {{harvnb|Van Inwagen|Sullivan|Bernstein|2023|loc=§ 4. The Methodology of Metaphysics}} }}</ref>

Possibility and necessity are further topics in ontology. Possibility describes what can be the case, as in "it is possible that [[extraterrestrial life]] exists". Necessity describes what must be the case, as in "it is necessary that three plus two equals five". Possibility and necessity contrast with actuality, which describes what is the case, as in "[[Doha]] is the capital of [[Qatar]]". Ontologists often use the concept of [[possible world]]s to analyze possibility and necessity.<ref>{{multiref | {{harvnb|Parent|loc=Lead Section}} | {{harvnb|Loux|Crisp|2017|pp=149–150}} | {{harvnb|Koons|Pickavance|2015|pp=154–155}} | {{harvnb|Mumford|2012|loc=§ 8. What Is Possible?}} }}</ref> A possible world is a complete and consistent way how things could have been.<ref>{{multiref | {{harvnb|Berto|Jago|2023|loc=Lead Section}} | {{harvnb|Pavel|1986|p=[https://books.google.com/books?id=OF7ZA54UpYYC&pg=PA50 50]}} }}</ref> For example, [[Haruki Murakami]] was born in 1949 in the actual world but there are possible worlds in which he was born at a different date. Using this idea, [[possible world semantics]] says that a sentence is possibly true if it is true in at least one possible world. A sentence is necessarily true if it is true in all possible worlds.<ref>{{multiref | {{harvnb|Menzel|2023|loc=Lead Section, § 1. Possible Worlds and Modal Logic}} | {{harvnb|Kuhn|2010|p=[https://books.google.com/books?id=c_eYe6NVaDMC&pg=PA13 13]}} }}</ref> The field of [[modal logic]] provides a precise formalization of the concepts of possibility and necessity.<ref>{{harvnb|Garson|2024|loc=Lead section, § 1. What is Modal Logic?}}</ref>

In ontology, [[Identity (philosophy)|identity]] means that two things are the same. Philosophers distinguish between qualitative and numerical identity. Two entities are qualitatively identical if they have exactly the same features, such as perfect identical twins. This is also called ''exact similarity'' and ''indiscernibility''. Numerical identity, by contrast, means that there is only a single entity. For example, if Fatima is the mother of Leila and Hugo then Leila's mother is numerically identical to Hugo's mother.<ref>{{multiref | {{harvnb|Kirwan|2005|pp=417–418}} | {{harvnb|Noonan|Curtis|2022|loc=Lead Section}} | {{harvnb|Kuhlmann|2010|pp=1867–1867b}} }}</ref> Another distinction is between synchronic and diachronic identity. Synchronic identity relates an entity to itself at the same time. Diachronic identity relates an entity to itself at different times, as in "the woman who bore Leila three years ago is the same woman who bore Hugo this year".<ref>{{multiref | {{harvnb|Gallois|2016|loc=§ 2.1 Diachronic and Synchronic Identity}} | {{harvnb|Noonan|Curtis|2022|loc=Lead Section, § 5. Identity Over Time}} }}</ref> The notion of identity also has a number of philosophical implications in terms of how it interacts with the aforementioned necessity and possibility. Most famously, Saul Kripke contended that ''discovered identities'' such as "Water is H<small>2</small>O" are necessarily true because "H<small>2</small>O" is what's known as a [[rigid designator]]. <ref>{{cite book |last1=Kripke |first1=Saul |title=Naming and Necessity |date=1980 |publisher=Harvard University Press, Blackwell |isbn=978-0-674-59845-4 |pages=128-129}}</ref>