Ontology

=== Identity ===
[[Identity (philosophy)|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 ''[[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 |last=Forrest |first=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 [[Max Black]], who describes a symmetrical universe consisting of only two spheres with the same features.<ref>{{cite journal |last=Black |first=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 |last=Cowling |first=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 |last=Gallois |first=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 [[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 [[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"/> [[Perdurantism]] or [[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 |last=Hawley |first=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 |last=Costa |first=Damiano |title=Persistence in Time |url=https://iep.utm.edu/per-time/ |website=Internet Encyclopedia of Philosophy |access-date=23 January 2021}}</ref>