"Now as to what concerns idea, if we consider them only in themselves and do not relate them to anything else beyond themselves, they cannot properly speaking be false ..." (IP3 51a)

"[T]he principal error and the commonest which we may meet with in [judgments], consists in my judging that the ideas which are in me are similar or conformable to the things which are outside me .." (IP3 51a)

Later Descartes offers a sense in which ideas can be false. There is the possibility of "material falsity": "i.e., when these ideas represent what is nothing as though it were something." (IP3 53b)