Descartes was a foundationalist. As we’ve seen, he required infallibility for beliefs that counted as genuine knowledge; these (supposedly) infallible beliefs acted for him as basic beliefs. We can see the connection between infallibility and foundational status. If a belief was fallible, then it might be wrong. If it might be wrong, one would need some assurance that it’s correct. Otherwise, there would be no difference between an acceptable belief and an unacceptable guess. But there won’t be other acceptable beliefs providing evidence in its favour if the belief is basic. So the only way a basic belief can be acceptable, it seems, is if it’s infallible. The most famous allegedly basic and infallible belief Descartes talks about is his belief that he exists. It’s infallible because he can’t be mistaken about it. No matter what other mistakes he makes, he can’t be mistaken about this one, because any sort of thinking, correct or mistaken, can only be done by an existing thinking being. He also supposes the existence of God to be demonstrable with certainty, infallibly; and next a variety of beliefs in the areas of mathematics and geometry –beliefs guaranteed infallible by God provided we follow the proper methodology in deriving them. Moreover, he claims infallibility of beliefs about the content of our own minds.