How Common Knowledge Is Possible

The two of us commonly know a proposition just in case we both know it, we both know that we both know it, we both know that we both know that we both know it, and so on. In a recent paper titled ‘Uncommon Knowledge’, Harvey Lederman argues against the possibility of common knowledge. His argument rests on the empirical claim that there are minor individual variations in how we perceive things. This motivates a principle about perception: when two people are perceiving something and it perceptually appears a certain way to one of them, then for all that person knows, it perceptually appears a slightly different way to the other. In this paper, I challenge Lederman’s perceptual principle and thereby his argument. In particular, I argue that there are some circumstances in which things perceptually appear a certain way to me and nonetheless I know that they don’t perceptually appear in a slightly different way to you. Indeed, I argue that not only are there exceptions to the perceptual principle, but they are widespread.