One of the foundational claims of Aristotle's Ethics is that virtue is “up to us and voluntary.” Provided that we have the appropriate moral education, we can mould our own characters by acting in the right ways. But the story is not as simple as it initially appears. In Politics VII.7, we are told that only some humans are “most easily led to virtue,” since they have natures that are ‘well-mixed.' Having a ‘well-mixed' nature turns out to be a matter of having blood that is hot, thin and pure since these material properties are most conducive to the development of spirit (which is required for courage) and intelligence (which is required for practical wisdom). Aristotle's emphasis on the importance of having the right kind of material nature, therefore, has significant implications for moral development. But some have worried that Aristotle's emphasis on material nature entails that the efficient cause of character is one's material nature (which is not up to us) and not, as he insists in the Ethics, decision (which is up to us). My aim in this paper is to show that we can resolve this tension by describing a person's potential to become virtuous in two different ways: in terms of passive potentiality (which is determined by the material in which the process of moral development takes place), and in terms of active potentiality (which is determined by decision which, acting as an efficient cause, works on the material properties of his blood).