The paper addresses the following question: why do human beings, on Aristotle’s view, have an innate tendency to badness, that is, to developing desires that go beyond, and often against, their natural needs? Given Aristotle’s teleological assumptions (including the thesis that nature does nothing in vain), such tendency should not be present. I argue that the culprit is to be found in the workings of rationality. In particular, it is the presence of theoretical reason that necessitates the limitless nature of (...) human non-rational desires. (shrink)