Possible Worlds: An Introduction to Logic and Its Philosophy

Talk of possible worlds is now a commonplace within philosophy. It began, nearly three hundred years ago, within philosophical theology. Leibniz thought it reassuring to say that although our world contains much that is evil, it is nonetheless the best of all possible worlds. Few philosophers today find this statement very plausible. Nevertheless, talk of the set of all possible worlds — when stripped of the suggestion that the actual world is better than any others — is nowadays frequently invoked as a means of illuminating other areas of philosophy. Ethics, epistemology, philosophy of psychology, philosophy of language, and — most notably of all — logic, are all benefiting from the insights of what is called "possible worlds semantics".
Unfortunately, most current talk of possible worlds is still regarded as the province of professionals; little of it has filtered down to those who are just beginning to learn their philosophy. Yet there is no good reason why this should be so. Although the higher reaches of possible-worlds semantics bristle with technical subtleties, its basic insights are really very simple. This book explains what those insights are and uses them to construct an integrated approach to both the philosophy of logic and the science of logic itself.