The best kind of tragedy concerns a person who is neither morally outstanding nor wicked, according to Aristotle in the Poetics. Tragedy should rather imitate a person who falls between these two extremes, and who comes to ruin through some kind of “failure” – in Greek: hamartia. It is clear that hamartia is the cause of the downfall of the tragic hero. However, Aristotle does not explain what he means by “hamartia”, and important questions about the tragic plot remain unanswered. What kind of “failure” is Aristotle referring to? Can the tragic hero be held morally responsible for his downfall? Or is it only a mishap that he cannot affect? In short: how are we to understand hamartia? Most contemporary philologists and philosophers hold that hamartia is brought about by a lack of important information, so that the failure is better explained as an “error of judgment”. By contrast, several earlier commentators understand hamartia as a “flaw of character”, and consider hamartia a moral shortcoming. In this thesis, I both defend the latter interpretation and reform it by emphasizing the intellectual aspects of hamartia. I argue (1) that the tragic hero comes to ruin due to a weakness of character (ἀκρασία) connected to one’s temper (θυμός), and (2) that this weakness is connected with the protagonist’s rationality by revealing that he lacks practical wisdom (φρόνησις).