More notes. Aristotle's Politics will follow shortly.
Aristotle begins with the assumption that everything in the world aims at some good: it has a purpose that is part of its definition. Objects can have multiple 'ends', which are not all of equal worth. The superior end is identified by its 'self-sufficiency': it is pursued for its own sake, rather than as a means to something better.
So what is the 'end' of a human being? What is 'good' for humans? Humans value happiness above everything else: it is self-sufficient, desired for its own sake. Honour, wealth and physical pleasure are means to attaining happiness, they are not ends in themselves.
Aristotle also postulates that everything in the world has certain capacities. A plant can grow. An animal can also perceive the world. Human beings can also reason. 'Virtue' is simply using these capacities well. Thus the excellent use of reason is what human beings should strive for. This is what is virtuous concerning human beings.
If we may bring in Thrasymachus from over here, you could ask whether the correct use of reason would necessarily lead to the identification of those virtues Aristotle eventually identifies. For example, is the virtue of justice always rational?
As we have seen, the highest human capacity is reason, and the highest human desire is happiness. These must be the same. Aristotle's ethical project is to make them the same: make the practice of virtue pleasant -- a part of a happy life. This requires education.
Aristotle is difficult because he claims that reason, virtue and happiness are intimately related, and yet at the same time human beings have to be conditioned to make this relation, which casts doubt over how natural or evident this relation is. You get the feeling that Plato's transcendentalism is not entirely abandoned here.
Aristotle applies the 'self-sufficiency' rule to work out what kind of happiness is superior. Reason directed towards others human beings (political life) is imperfect because people are changeable. Reason applied to unchangeable objects (science / philosophy) offers a more sustained and intense pleasure. Men engaged in such activity are God-like: contemplative and self-sufficient.
If the ultimate end of human beings is scientific enquiry (often described in religious language by Aristotle), the reason-virtue-happiness relation may make more sense. Practical reason, a virtuous character, and the pleasant state of mind it produces, are all instrumental: they ensure the stable conditions for the exercise of the highest human capacity (reason) towards the most perfect object available to it: the unchanging laws of the universe.
So while Plato insists a happy community requires the rule of philosophers, Aristotle insists that philosophers being free to be philosophers is what constitutes a happy community: a rather more authoritarian attitude.