Editing Metaphysics (section)

=== Possibility and necessity ===
The concepts of possibility and necessity convey what can or must be the case, expressed in modal statements like "it is possible to find a cure for cancer" and "it is necessary that two plus two equals four". Modal metaphysics studies metaphysical problems surrounding possibility and necessity, for instance, why some modal statements are true while others are false.<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>{{efn|A further topic concerns different types of modality, such as the contrast between physical, metaphysical, and logical necessity based on whether the necessity has its source in the [[Scientific law|laws of nature]], the essences of things, or the laws of logic.<ref>{{multiref | {{harvnb|Hanna|2009|p=[https://books.google.com/books?id=ka9BhOL1ev8C&pg=PA196 196]}} | {{harvnb|Hale|2020|p=[https://books.google.com/books?id=OSvsDwAAQBAJ&pg=PA142 142]}} }}</ref>}} Some metaphysicians hold that modality is a fundamental aspect of reality, meaning that besides facts about what is the case, there are additional facts about what could or must be the case.<ref>{{multiref | {{harvnb|Goswick|2018|pp=97–98}} | {{harvnb|Wilsch|2017|pp=428–429, 446}} }}</ref> A different view argues that modal truths are not about an independent aspect of reality but can be reduced to non-modal characteristics, for example, to facts about what properties or linguistic descriptions are compatible with each other or to [[Fictionalism|fictional statements]].<ref>{{multiref | {{harvnb|Goswick|2018|pp=97–98}} | {{harvnb|Parent|loc=§ 3. Ersatzism, § 4. Fictionalism}} | {{harvnb|Wilsch|2017|pp=428–429}} }}</ref>

Borrowing a term from German philosopher [[Gottfried Wilhelm Leibniz]]'s [[theodicy]], many metaphysicians use the concept of [[possible world]]s to analyze the meaning and ontological ramifications of modal statements. A possible world is a complete and consistent way the totality of things could have been.<ref>{{multiref | {{harvnb|Menzel|2023|loc=Lead Section, § 1. Possible Worlds and Modal Logic}} | {{harvnb|Berto|Jago|2023|loc=Lead Section}} | {{harvnb|Pavel|1986|p=[https://books.google.com/books?id=OF7ZA54UpYYC&pg=PA50 50]}} | {{harvnb|Campbell|2006|loc=§ Possible Worlds}} }}</ref> For example, the [[dinosaurs]] were [[Cretaceous–Paleogene extinction event|wiped out]] in the actual world but there are possible worlds in which they are still alive.<ref>{{harvnb|Nuttall|2013|p=[https://books.google.com/books?id=luc-Hf_bEOIC&pg=PT135 135]}}</ref> According to possible world semantics, a statement is possibly true if it is true in at least one possible world, whereas it 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> [[Modal realism|Modal realists]] argue that possible worlds exist as concrete entities in the same sense as the actual world, with the main difference being that the actual world is the world we live in while other possible worlds are inhabited by [[Counterpart theory|counterparts]]. This view is controversial and various alternatives have been suggested, for example, that possible worlds only exist as abstract objects or are similar to stories told in works of [[fiction]].<ref>{{multiref | {{harvnb|Parent|loc=Lead Section, § 2. Lewis' Realism, § 3. Ersatzism, § 4. Fictionalism}} | {{harvnb|Menzel|2023|loc=Lead Section, § 2. Three Philosophical Conceptions of Possible Worlds}} | {{harvnb|Campbell|2006|loc=§ Modal Realism}} }}</ref>