David Chalmers maintains that certain kinds of conceivability are reliable guides to metaphysical possibility. In this essay, I argue that Chalmers is wrong about the relationship between conceivability and metaphysical possibility. I begin by explaining the notion of metaphysical possibility and by distinguishing it from those of other kinds of so-called possibility (including epistemic, logical, conceptual, physical, and natural possibility). I then explain Chalmers’s view on the relationship between conceivability and metaphysical possibility—a view that he calls modal rationalism. Next, I examine his positive case for his view, and I argue that it is unpersuasive. In particular, I argue, his attempt to show that modal rationalism is a priori is clearly a failure: given a proper understanding of the notion of metaphysical possibility, the balance of reasons overwhelming favors the hypothesis that modal rationalism is not a priori, and we are entitled to assume that it is not a priori unless compelling reasons to think otherwise are revealed. Finally, I produce two arguments that show that because modal rationalism is not a priori, it is false. I conclude that since there is every reason to think that modal rationalism is not a priori and no plausible reason to think that it is a priori, and since the falsity of modal rationalism follows from its not being a priori, there is every reason to think that modal rationalism is false.