 A second objection is suggested by Peirce’s reference to a “doubt in our hearts.” Here, we should distinguish two kinds of doubt, in terms of two kinds of ways that doubt can defeat knowledge. Some doubts purport to undermine one’s conviction or belief – call these ‘belief-defeating’ doubts. Other doubts purport to undermine one’s justification (whether or not they undermine belief) – call these ‘justification-defeating’ doubts. What Peirce calls a ‘doubt in our hearts’ is strongly suggestive of a belief-defeating doubt. Is Peirce therefore right that only belief-defeating doubts can undermine knowledge? Longstanding traditions in philosophy acknowledge that there may be truths we believe in our hearts (as it were), but which we do not know. This is one of the intended lessons of methodical doubt. The sceptical scenarios are supposed to help us appreciate that though we believe that 2+3=5, and believe that we’re awake, and believe there’s an external world, we may nonetheless lack perfect knowledge. As already noted, Descartes writes of external world doubt that “no sane person has ever seriously doubted” such matters – a remark perhaps best understood in terms of belief-defeating doubt. Justification-defeating doubts are sufficient to undermine perfect knowledge, and this is the sort of doubt put forward in the First Meditation.