 Once the existence of a distinctively metaphysical form of propositional necessity is accepted, it is natural to wonder whether it is possible to say more about its nature. Kit Fine (1994) offers an account of it that appeals to the traditional distinction between those properties of a thing that it possesses by its very nature and those that it has merely accidentally. For example, it lies in the nature of water to be composed of hydrogen and oxygen—being composed in this way is part of what it is to be water—but it is merely accidental to water that we use it to brush our teeth. A proposition is metaphysically necessary just in case it is true in virtue of the natures of things. (Also see Kment 2014, chs. 6–7, and Ditter 2020.) Other philosophers (Rayo 2013, §2.2.1, ch. 5; Dorr 2016) have discussed the idea that metaphysical necessity can be explained in terms of the idiom “To be \(F\) is to be \(G\)” (as in “To be water is to be H2O”). Yet another account ties the metaphysical notion of necessity constitutively to causation and explanation (Kment 2006a,b, 2014, 2015a; also see the exchange between Lange 2015 and Kment 2015b).