This book, one of the first full-length studies of the modalities to emerge from the debate to which Saul Kripke, David Lewis, Ruth Marcus, and others are contributing, is an exploration and defense of the notion of modality de re, the idea that objects have both essential and accidental properties. Plantinga develops his argument by means of the notion of possible worlds and ranges over such key problems as the nature of essence, transworld identity, negative existential propositions, and the existence (...) of unactual objects in other possible worlds. He also applies his logical theories to the elucidation of two problems in the philosophy of religion: the problem of evil and the ontological argument. (shrink)

I am going to describe a variant formulation of classical extensional first-order logic and contrast it with the standard formulation. The formulation I will give is in one clear sense equivalent to the standard one, and it is a routine task to show that it is equivalent to it in this sense. So one might regard my formulation as a mere notational variation. But there are also ways in which the two formulations I will contrast are not equivalent, and I (...) will argue that these ways reflect an important conceptual difference between them. The differences become more important as one generalizes or extends classical logic in various ways, but they are there even in the classical systems. (shrink)