Editing Metaphysical necessity (section)

== Hume's dictum ==
'''Hume's dictum''' is a thesis about necessary connections between distinct entities. Its original formulation can be found in [[David Hume]]'s [[A Treatise of Human Nature]]: "There is no object, which implies the existence of any other if we consider these objects in themselves".<ref name="Hume">{{cite book |last1=Hume |first1=David |title=A Treatise of Human Nature |date=1739 |publisher=The Project Gutenberg |url=https://www.gutenberg.org/files/4705/4705-h/4705-h.htm |chapter=Book I, Part III, Section VI |access-date=2024-04-30 |archive-date=2023-05-11 |archive-url=https://web.archive.org/web/20230511124134/http://gutenberg.org/files/4705/4705-h/4705-h.htm |url-status=live }}</ref> Hume's intuition motivating this thesis is that while experience presents us with certain ideas of various objects, it might as well have presented us with very different ideas. So when I perceive a bird on a tree, I might as well have perceived a bird without a tree or a tree without a bird. This is so because their [[Essence|essences]] do not depend upon another.<ref name="Hume"/> [[David Lewis (philosopher)|David Lewis]] follows this line of thought in formulating his ''principle of recombination'': "anything can coexist with anything else, at least provided they occupy distinct spatiotemporal positions. Likewise, anything can fail to coexist with anything else".<ref>{{cite book |last1=Lewis |first1=David |title=On the Plurality of Worlds |date=1986 |publisher=Wiley-Blackwell |page=88 |url=https://philpapers.org/rec/LEWOTP-3 |access-date=2024-04-30 |archive-date=2023-06-23 |archive-url=https://web.archive.org/web/20230623011828/https://philpapers.org/rec/LEWOTP-3 |url-status=live }}</ref>

Hume's dictum has been employed in various arguments in contemporary [[metaphysics]]. It can be used, for example, as an argument against nomological necessitarianism, the view that the [[Law of nature (science)|laws of nature]] are necessary, i.e. are the same in all possible worlds.<ref>{{cite book |last1=Gibbs |first1=Cameron |title=A Defense of Hume's Dictum |date=2019 |url=https://scholarworks.umass.edu/dissertations_2/1733/ |chapter=6. THE ARGUMENT FROM PLENITUDE |access-date=2024-04-30 |archive-date=2023-06-26 |archive-url=https://web.archive.org/web/20230626043758/https://scholarworks.umass.edu/dissertations_2/1733 |url-status=live }}</ref><ref>{{cite encyclopedia |last1=Swartz |first1=Norman |title=Laws of Nature |url=https://iep.utm.edu/lawofnat/ |access-date=28 November 2020 |archive-date=28 August 2023 |archive-url=https://web.archive.org/web/20230828072405/https://iep.utm.edu/lawofnat |url-status=live }}</ref> To see how this might work, consider the case of salt being thrown into a cup of water and subsequently dissolving.<ref>{{cite journal |last1=Bird |first1=Alexander |title=Necessarily, Salt Dissolves in Water |journal=Analysis |date=2001 |volume=61 |issue=4 |pages=267–274 |doi=10.1111/1467-8284.00304 |url=https://philpapers.org/rec/BIRNSD |access-date=2024-04-30 |archive-date=2023-06-23 |archive-url=https://web.archive.org/web/20230623011704/https://philpapers.org/rec/BIRNSD |url-status=live }}</ref> This can be described as a series of two events, a throwing-event and a dissolving-event. Necessitarians hold that all possible worlds with the throwing-event also contain a subsequent dissolving-event. But the two events are distinct entities, so according to Hume's dictum, it is possible to have one event without the other. An even wider application is to use Hume's dictum as an axiom of modality to determine which propositions or worlds are possible based on the notion of recombination.<ref>{{cite book |last1=Gibbs |first1=Cameron |title=A Defense of Hume's Dictum |date=2019 |url=https://scholarworks.umass.edu/dissertations_2/1733/ |chapter=2. CONSTRAINTS ON FORMULATING HUME’S DICTUM |access-date=2024-04-30 |archive-date=2023-06-26 |archive-url=https://web.archive.org/web/20230626043758/https://scholarworks.umass.edu/dissertations_2/1733 |url-status=live }}</ref><ref>{{cite encyclopedia |last1=Menzel |first1=Christopher |title=Possible Worlds |url=https://plato.stanford.edu/entries/possible-worlds/  |publisher=Metaphysics Research Lab, Stanford University |access-date=28 November 2020 |date=2017}}</ref>

[[Ludwig Wittgenstein]] likewise critiqued the notion of 'metaphysical neccessity' writing that "the sun will rise to-morrow, is an hypothesis; and that means that we do not know whether it will rise. A necessity for one thing to happen because another has happened does not exist. There is only logical necessity".<ref>{{cite book |last1=Wittgenstein |first1=Ludwig |title=Tractatus Logico-Philosophicus |date=1921}}</ref>