 Interestingly, Plutarch does not refer to the Timaeus to support his theory of the tripartite soul, but rather to Aristotle (De virtute morali 442B-C), whom Plutarch, like most ancient and modern commentators, recognizes as adopting essential aspects of Plato's doctrine. The essential feature that Aristotle shares with Plato is the belief in rational and non-rational aspects of the soul. This accounts for unself-controlled actions that, Plutarch thinks, prove how mistaken is the Stoic conception of human agency as deriving from reason alone. Plutarch also relies largely on Aristotle's Nicomachean Ethics with regard to the nature of virtue. He describes virtue as being an extreme of excellence (akrotês), which however lies in a mean, determined by reason, between two opposite emotions (De virtute morali 443D-444D; Nicomachean Ethics 1107a6-8). Plutarch argues that there can be no virtue without some emotion. Without some amount of fear, Plutarch contends, there can be no courage, for instance (De virtute morali 451E-452A); courage, he claims, is the virtue that one acquires when, in a state of fear, one manages to subordinate fear to a goal set by reason, such as fighting for one's country; in this sense, emotion is an ally to reason in constituting virtue (cf. De tranq. animi 471D). Plutarch suggests this specifically in the case of anger becoming bravery (De ira fr. 148 Sandbach). Anger, when moderated and guided by reason, can also motivate reasonable and due vengeance (De sera 551A-B).