Is mathematical knowledge a precedent for modal knowledge?: A novel objection to Lewis’s modal epistemology

Abstract The goal of this paper is to raise a novel objection to Lewis’s modal realist epistemology. After reformulating his modal epistemology, I shall argue that his view that we have necessary knowledge of the existence of counterparts ends up with an absurdity. Specifically, his analogy between mathematical knowledge and modal knowledge leads to an unpleasant conclusion that one’s counterpart exists in all possible worlds. My argument shows that if Lewis’s modal realism is true, we cannot know what is possible. Conversely, if we can know what is possible, his modal realism is false. In the remainder of the paper, I shall consider and block possible objections to my argument.