 To this last question, many foundationalists reply: experience (we are talking here about empirical knowledge; a priori knowledge raises interesting problems of its own, and it is of course also subject to Agrippa’s trilemma). To a rough first approximation that glosses over many important philosophical issues, experiences are mental states that, like beliefs, aim to represent the world as it is, and, like beliefs too, can fail in achieving that aim—that is, experiences can misrepresent. Nevertheless, experiences are not to be identified with beliefs, for it is possible to have an experience as of, e.g., facing two lines that differ in length without having the belief that one is facing two lines that differ in length—a combination of mental states that anyone familiar with the Müller-Lyer illusion will recognize.