Thesis: Our characteristics, virtue and vice, are voluntary.
The end is the object of wish, and the means to the end are the objects of deliberation and choice (EN III.5.1113b5). Voluntary actions are those where the agent is the source of motion and the agent acts knowingly, i.e. not under constraint (1.1110a15). Therefore, actions concerned with means are based on choice and are voluntary actions (5.1113b6).
The activities that deal with means are where the virtues and vices express themselves. Therefore, virtue and vice depend on ourselves. This is because, when it is in our power to act, it is also in our power not to act. Thus, if we have the power to take action when it is good to, we have the power to act where not taking action is bad (10).
It follows that, since we have power over the goodness or badness of our actions, we must conclude that it depends on us whether we are virtuous or vicious individuals (15).
We must also clarify that our actions and our characteristics are not voluntary in the same sense. That is, we are in control of our actions from beginning to end, but we control only the beginning of our characteristics. This is because the power to behave or not to behave in a given way was ours in the first place, but, since each step in developing a characteristic is imperceptible, we have no control over the development outside of the initial action (1115a4).
There are, however, people who think that we are not the source of our own goodness or badness.
First, one might object that "no one is voluntarily wicked nor involuntary happy" (15). But this is not entirely true because one cannot be happy as a result of involuntary actions because rational activity, which is necessarily voluntary, is a necessary condition of happiness. On the other hand, wickedness must be voluntary because, if we do not accept that, we must deny that a man is the source of his actions, just as a father is the source of his children (18).
Second, someone might object that "all men seek what appears good to them, but they have no control over how things appear to them; the end appears different to different men" (32). If this is true, then no one is responsible for his own wrongdoing, thus ethical inquiry is pointless. On the other hand, if the individual is somehow responsible for his own characteristics, he is similarly responsible for what appears good to him (b1).
Yet the end that we aim for does not appear to be something we choose for ourselves, instead, it seems to depend on a kind of natural gift that grants one proper vision, a kind that allows one to make correct judgements and choose what is truly good (8).
But if this theory is true, virtue cannot be any more voluntary than vice. This is because the end would already be determined by nature, i.e. through whatever kind of vision nature bestows upon us. Thus, it cannot be the case that nature alone determines what man takes to be good, but is, to some extent, due to himself. Even if the end is given by nature, virtue is voluntary in the sense that a good man performs the actions that lead up to the end voluntarily (20).