Abstract

In this thesis, the modal realist metaphysics of modality of David Lewis is subjected to a Kantian critique. Modal realism is the theory that possible worlds are the sorts of things our actual world is. Lewis's argument for modal realism in "Possible Worlds" contains what in Kantian terms are synthetic apriori propositions. Kant's answer to the question of how synthetic apriori modal propositions are possible is examined. Kant is found to have erred in choosing de dicto modal concepts and principles, given his notion of an objective transcendental deduction, an argument which shows that, without these concepts, experience of objects would not be possible. The error is corrected by supplying de re modal concepts and a new modal principle: the principle of essence and accident. I argue that the principle of essence and accident is a valid synthetic apriori modal principle. Lewis's argument for possible worlds is shown to be fallacious by pointing out that synthetic apriori principles do not imply the existence of transcendent entities. However, I put forth a "practical grounds argument"--an argument from the necessary conditions of moral practice--for "postulating" concrete possible worlds. Finally, I propose a concept of concrete possible worlds that is more compatible with Kantian principles than is Lewis's conception