The knowledge argument is something that is both an ideal for philosophy and yet surprisingly rare: a simple, valid argument for an interesting and important conclusion, with plausible premises. From a compelling thought experiment and a few apparently innocuous assumptions, the argument seems to give us the conclusion, a priori, that physicalism is false. Given the apparent power of this apparently simple argument, it is not surprising that philosophers have worried over the argument and its proper diagnosis: physicalists have disputed its validity, or soundness or both; in response, non-physicalists have attempted to reformulate the argument to show its real anti-physicalist lesson.