 Possibilism would almost surely not be taken as seriously as it is were it not for the dramatic development of possible world semantics for modal logic in the second half of the twentieth century. For it not only enables the possibilist to formulate truth modal conditions with particular clarity and cogency, it yields a natural and elegant quantified modal logic, known as SQML, in which possibilism’s fundamental metaphysical principles fall out as logical truths. In order to appreciate the cogency of possibilism, therefore, it is important to understand basic possible world semantics.