Dennett is one of the few philosophers who think that Mary could work out what it would be like to see colours. In some places he seems to sketch out how this might be done. I argue that his account depends on attributing to Mary knowledge that she does not possess, namely knowledge of her own current brain state. Dennett denies that is suggesting any way in which Mary might work it out, as he is merely presenting an alternative intuition and thought experiment, and develops this with his story of Swamp Mary. I claim that there are good reasons for rejecting this thought experiment. He confuses allowing Mary to put herself in the physical state in which someone would be who had had the experience with working out what the experience would be like on the basis of scientific knowledge alone. Dennett’s appeal to Mary’s vast knowledge of physiology is also inconsistent with his claim that mental states are no more than dispositions.