In this paper I offer a close reading of Aristotle’s argument in the Nicomachean Ethics 3.5.1114a31–b25 and try to show that despite considerable interpretive difficulties, some clear structure can nevertheless be discerned. While Aristotle’s main concern in this passage is to refute the so-called asymmetry thesis – the thesis that virtue is voluntary, but vice is not – there is much more in it than just a dialectical encounter. Aristotle wants to respond to a more general objection, which has as its target the voluntariness of both virtue and vice, and which is provoked by some of his ideas in EN 3.4 and 3.5. Further, I will try to show why Aristotle thinks that we are only co-causes ( sunaitioi ) of our dispositions. In my opinion, his reasons have nothing to do with compatibilist or incompatibilist considerations as they are commonly understood in modern philosophy. In particular, he does not want to argue that nature (as well as social environment, early educators, etc.) is aitios of our dispositions just as ourselves are. Rather, we are co-causes of our dispositions because we are (efficient) causal origins of actions without which a certain good, which is the final cause of our actions and of our dispositions, cannot be achieved. Finally, I will try to show that Aristotle’s discussion implies that there is no more to the responsibility for dispositions than there is to the responsibility for actions.