Now as far as ideas are concerned, if they are considered alone in their own right, without being referred to something else, they cannot, properly speaking, be false. For whether it is a she-goat or a chimera that I’m imagining, it is no less true that I imagine the one than the other. Moreover, we need not fear that there is falsity in the will itself or the affects, for although I can choose evil things or even things are utterly non-existent, I cannot conclude from this that it is untrue that I do choose these things. Thus there remain only judgments in which I must take care not to be mistaken. Now the principal and most frequent error to be found in judgments consists in the fact that I judge that the ideas which are in me are similar to or in conformity with certain things outside me. Obviously, if I were to consider these ideas merely as certain modes of my thought, and were not to refer them to anything else, they could hardly give me any subject matter for error. (37)
Descartes, René. Meditations on First Philosophy. Trans. Donald A. Cress. Hackett Publishing 1998.