Essentialist theses claim that certain kinds of properties are essential to certain kinds of individuals. For example, according to origin essentialism, biological individuals have their biological origins essentially. Origin essentialism therefore requires that an individual like George W. Bush have his actual biological origins—in this case, Barbara and George Bush—in any possible world in which it exists. (On origin essentialism, see Kripke (1979) and Robertson (1998). On other essentialisms, see Cartwright (1968) and Mackie (2006).)