If, therefore, there is some end of our actions that we wish for on account of itself, the rest being things we wish for on account of this end, and if we do not choose all things on account of something else—for in this way the process will go on indefinitely such that the longing involved is empty and pointless—clearly this would be the good, that is, the best. (1.2.1094a19-23)
Aristotle explains his idea of the most "choiceworthy" thing. He doesn't say specifically what it is at this point (why would you keep reading?), but he does make it clear that we yearn for the good that's most complete. When we stumble onto that certain something that doesn't need anything else to make it better, we should choose that as the greatest possible good in life.
And the person lacking self-restraint acts out of desire, but he does not do so from choice; the self-restrained person, conversely, acts from choice but not out of desire. And whereas desire opposes choice, desire does not oppose desire. Desire has to do with what is pleasant and painful, whereas choice has to do with neither the painful nor the pleasant. (3.2.1111b14-17)
Aristotle begins to narrow the scope of what is truly voluntary by defining the types of actions subject to choice. Desire, it seems, has the upper hand in each of these situations. Because desire comes from a different place than rational thought, it doesn't easily obey our intellect. And without intellect, Aristotle will later tell us, there's no real capacity to deliberate and choose.
What sort of thing is choice, then, since it is none of the things mentioned? It indeed appears to be something voluntary, but not everything voluntary is an object of choice. But is it, therefore, at least an object of prior deliberation? For choice is accompanied by reason and thought. (3.2.1112a13-16)
The answer is yes: choice specifically is the product of prior deliberation. He'll go further to say that we can't properly choose if we are ignorant or without virtue. While this definitely takes away some personal responsibility from those who do stupid things, it doesn't let them off the hook. The inability to choose may keep us from being truly unjust, but it doesn't make us good or honorable, and therefore excludes us from happiness.
Since what is chosen is a certain longing, marked by deliberation, for something that is up to us, choice would in fact be a deliberative longing for things that are up to us. For in deciding something on the basis of having deliberated about it, we long for it in accord with our deliberation (3.3.1113a10-13)
This is choice in a nutshell. Note that in order to choose properly, three things have to be present: 1) longing (preferably correct longing); 2) deliberation, or the ability to think before acting; 3) the ability to act under our own steam, not out of compulsion. If any of these are missing or incomplete, we won't act out of choice but out of compulsion, desire, or necessity.
In this way too, it was possible at the beginning for both the unjust person and the licentious one not to become such as they are, and hence they are what they are voluntarily; but once they become such, it is no longer possible for them to be otherwise. (3.5.1114a19-22)
Choice plays a huge role in the development of our virtues. When we're in the position to deliberate and choose our actions (i.e. what we do is voluntary) we're choosing also the type of person we are becoming. If we choose poorly, we're habituating ourselves to become bad people. And once we walk down that path, there's no turning back because eventually, our reason (and hence, our ability to choose properly) will be destroyed.
(It is not in making having the capacity to boast but in making the choice to do so that someone is a boaster, for choice accords with one's characteristic, and he is a boaster because he is that sort of person). (4.7.1127b14-16)
It's important to remember that characteristics or virtues can't shape our personality unless they're active in us (i.e. they are translated into action). We can't be called a good person until we actually do things that are good. It also works the same way for the annoying stuff. While we all have the capacity to boast, it's the choice to do so that motivates us to action—and therefore gets us pegged as an obnoxious exaggerator.
Some voluntary things we do because we have chosen them, others we do though we have not chosen them: we choose all those that we deliberated about beforehand, and those not chosen are those not deliberated about beforehand. (5.8.1135b9-11)
It's important especially for lawgivers to understand what types of actions are voluntary and which are involuntary, Aristotle says. For one thing, it isn't always easy to determine this. Even when we can categorize an action one way or another, there are often circumstances that make our actions quite a mixed bag.
An action might be considered voluntary (i.e. having its origin within ourselves) but not be the result of pre-meditation or careful mulling over the consequences (i.e. deliberation). To call something truly voluntary, the key thing is choice: conscious deliberation about our course of action.
But if a person harms someone from choice, he acts unjustly; and it is in reference to these acts of injustice that he who does them is himself unjust, whenever the act is contrary to what is proportional or equal. Similarly too a person is himself just whenever he performs a just act, having chosen to do so...(5.8.1136a2-3)
Aristotle differentiates between a person who does an unjust act (but is not an unjust person) and one who does an unjust act and actually is unjust (and applies the same thinking to just acts).
It may seem like a lot of hair-splitting, but the distinction's important. We might do something wrong that ends in harm (i.e. running a stop sign), but it doesn't mean that we acted with the intention to harm. In this case, our actions are unjust, but we aren't. Choice (or prior deliberation) is crucial in determining such intent. And it's important to remember that we can't accidentally be just or unjust people.
It is manifest from this also who the equitable person is: he who is disposed to choose and to do these sorts of things and is not exacting to a fault about justice, but is instead disposed to take less for himself even though he has the law on his side, is equitable. (5.10.113734-1138a1)
Aristotle admits to us that the law isn't perfect where justice is concerned, especially in specific cases that require a judgment not exactly covered by law. Equity is the characteristic (or virtue) that allows a person to be equitable—or have greater concern for Justice (with a capital "J"—general justice)—rather than legalistic. It's clear who Aristotle thinks is a more superior person.
Of action, then, choice is the origin—that from which the motion arises but not that for the sake of which one moves; and of choice, the origin is one's longing and the reasoning that indicates what it is for the sake of which one acts. Hence there cannot be choice either in the absence of intellect and thinking or in the absence of moral characteristic, for there cannot be acting well or its contrary in action in the absence of thinking and character. (6.2.1139a31-36)
Some might argue that we can act perfectly well without thinking. Aristotle would clearly disagree. In this chain of command, we can see that choice is rooted in both thought and virtue. We sift through opinions about a course of action when we deliberate about how best to reach the desired end—which should be shaped by virtuous longing.
"Snap judgments" would not, then, be considered the product of proper choice. By further defining how choice works, Aristotle's also rapidly narrowing our perception of what kinds of actions are voluntary…since choice is a crucial component of intentional behavior.