 Descartes (1641/1984) famously endorsed the indubitability of “I think”, which he extends also to such mental states as doubting, understanding, affirming, and seeming to have sensory perceptions. He also appears to claim that the thought or affirmation that I am in such states is infallibly true, at least if that thought is clear and distinct. He was followed in this—especially in his infallibilism—by Locke (1690 [1975]), Hume (1739 [1978]), twentieth-century thinkers such as Husserl (1913 [1982]), Ayer (1936 [1946], 1963), Lewis (1946), and the early Shoemaker (1963), and many others. Historical arguments for indubitability and infallibility have tended to center on intuitive appeals to the apparent impossibility of doubting or going wrong about such matters as whether one is having a thought with a certain content or is experiencing pain or having a visual experience as of seeing red.