Abstract

Ruth Barcan Marcus has argued that, just as we cannot know what is false, we cannot believe what is impossible.1 I will offer an interpretation of her defense of this view. I will then argue, first, that if the defense succeeded it would also justify rejecting many, perhaps most, of our ordinary belief ascriptions; and second, that, luckily, the defense does not succeed. Finally, I suggest that despite its failure there is something correct and important in Marcus's argument.