Despite the humor noted to the left, Aristotle is certainly not being humorous in book 2. When it comes to trying to figure out the right way to live, Aristotle tried to set down a set of guidelines that would help any thoughtful person.
The rest of book 2 is an exercise in defining what virtue is. At the end of chapter 4, Aristotle makes clear that even if someone is a philosopher, to know what is right is not enough. In order to be just, we must (all of us) practice justice. In other words, we must perform the act to be what we are trying to become. Somehow, action makes the soul better, and it has a hand in forming the order of the soul. Presumably, we must first know what the right action is, but to make it real, we must act on it, which has the added effect of making our soul more in line with our reason (or perhaps he means to say reason more in charge of the different parts of the soul).
Chapter 5 thus speaks to the different parts of the soul in relation to acting virtuously. The Sachs translation makes the division clear. We have feelings, and we are predisposed to feel one thing or another pending the circumstance, but it is our “active condition” (once again the working of the soul) which either does well or ill. We may be predisposed to feel, say, anger, but our active condition works on not acting on that feeling. The action, then, is related to virtue or vice, not the feeling itself.
Virtue is an “active condition” and a certain sort of one at that. It is a mean between the extremes. While not everything has a mean condition, we must keep in mind that not everyone will need to practice the same amount of mean. For example, the proper amount of food for one, may be not proper for another.
Nevertheless, the mean as it pertains to virtue is of most importance, and more so than forcing ourselves to do the right thing, we should endeavor to feel the proper thing when we ought and in the manner we ought. All of this is very difficult. We may miss the target, and we are likely to miss it. Missing the target is easy.
For the remainder of Book 2, Aristotle lists the extremes and means. We should mark out what mean to attain in each case.