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 have contributed, is an exploraton and defence of the notion of modality "de re," the idea that objects have both essential and accidental properties. The arguement is developed by means of the notion of possible worlds and ranges over key problems including the nature of essence, trans-world identity, negative existential propositions, and the existence of unactual objects in other possible worlds. In the final chapters Professor Plantinga applies his logical theories to the elucidation of two problems in the philosophy of religion: the Problem of Evil and the Ontological Arguement. The first of these, the problem of reconciling the moral perfection and omnipotence of God with the existence of evil, can, he concludes, be resolved, and the second given a sound formulation. The book ends with an appendix on Quine's objection to quantified modal logic.