In Book III we will get our first fully described virtue, courage. Courage is not the most important virtue for Aristotle -- indeed, it is not even one of his major virtues -- but it is the easiest to conceptualize. It therefore serves as a useful model for the more theoretical ones.
First, however, he wants to say a few more words about the importance of choice on what is or isn't virtue. He has already said once that virtue is only about things where we make a choice; here he expands on that by considering some things that can make our actions compelled or involuntary.
Since virtue is concerned with passions and actions, and on voluntary passions and actions praise and blame are bestowed, on those that are involuntary pardon, and sometimes also pity, to distinguish the voluntary and the involuntary is presumably necessary for those who are studying the nature of virtue, and useful also for legislators with a view to the assigning both of honours and of punishments. Those things, then, are thought-involuntary, which take place under compulsion or owing to ignorance; and that is compulsory of which the moving principle is outside, being a principle in which nothing is contributed by the person who is acting or is feeling the passion, e.g. if he were to be carried somewhere by a wind, or by men who had him in their power.
Emphasis added. This notion of the principle of action being outside is exactly parallel to his model in Physics II.1: "Of things that exist, some exist by nature, some from other causes." These 'things' include motions, so if a motion of yours (e.g. an action) exists, either it was caused by your own nature, or it was caused by something else. If it was caused by your nature -- your quest for food, or love, or honor -- then it was an action of yours that must be judged (although it could still have been done in ignorance). If it was caused by something else, like men forcing you as their prisoner, then it was not your action at all: it was involuntary.
But with regard to the things that are done from fear of greater evils or for some noble object (e.g. if a tyrant were to order one to do something base, having one's parents and children in his power, and if one did the action they were to be saved, but otherwise would be put to death), it may be debated whether such actions are involuntary or voluntary.
As already mentioned, Aristotle will endorse a notion of justice that requires laws that compel virtue; but we will also see that he doesn't value such actions as real examples of the virtue. Hector's courage, for example, he will compare to a soldier whose action on the battlefield is driven by the law rather than by an inner drive; being at least partly externally compelled, it doesn't count for as much in the final judgment.
Something of the sort happens also with regard to the throwing of goods overboard in a storm; for in the abstract no one throws goods away voluntarily, but on condition of its securing the safety of himself and his crew any sensible man does so. Such actions, then, are mixed, but are more like voluntary actions; for they are worthy of choice at the time when they are done, and the end of an action is relative to the occasion. Both the terms, then, 'voluntary' and 'involuntary', must be used with reference to the moment of action. Now the man acts voluntarily; for the principle that moves the instrumental parts of the body in such actions is in him, and the things of which the moving principle is in a man himself are in his power to do or not to do. Such actions, therefore, are voluntary, but in the abstract perhaps involuntary; for no one would choose any such act in itself.
Emphasis added. As in I.3, where we were talking about things that are true "probably" or "for the most part," here too we end up having to make some pragmatic distinctions. It's not a binary: some things are sort-of voluntary, or closer-to voluntary, but there are elements of the involuntary present, unchosen considerations like the storm.
For such actions men are sometimes even praised, when they endure something base or painful in return for great and noble objects gained; in the opposite case they are blamed, since to endure the greatest indignities for no noble end or for a trifling end is the mark of an inferior person. On some actions praise indeed is not bestowed, but pardon is, when one does what he ought not under pressure which overstrains human nature and which no one could withstand. But some acts, perhaps, we cannot be forced to do, but ought rather to face death after the most fearful sufferings; for the things that 'forced' Euripides Alcmaeon to slay his mother seem absurd. It is difficult sometimes to determine what should be chosen at what cost, and what should be endured in return for what gain, and yet more difficult to abide by our decisions; for as a rule what is expected is painful, and what we are forced to do is base, whence praise and blame are bestowed on those who have been compelled or have not.
After the jump, more on how to decide what is compulsory, is not, and how to balance the judgments. I won't break this chapter up into multiple parts; it's fairly straightforward.