 Versions of this entry have been presented at the Arché Metaphysics and Logic research seminar at the University of St Andrews, at the Logic of Conceivability seminar at the University of Amsterdam, and at the 2020 Hamburg Summer School, at the Institute of Philosophy of the University of Hamburg. We are grateful to the participants, as well as Sara Bernstein, Max Cresswell, Greg Restall and two referees of the Stanford Encyclopedia of Philosophy, for their valuable feedback. Franz Berto’s research for this entry has been funded by the European Research Council (ERC CoG), Grant Number 681404.