Some notes (those in black are roughly exegesis, in red are my thoughts/interpretations):
First, while in Plato we find a "radical and sound alternative" to Athenian democracy (the same democracy that exhausted itself in a 30 year war with Sparta and executed its greatest thinker, Socrates), in Aristotle we instead find a detached and conservative ethics. Aristotle's work is less a critique of his society and more an attempt to raise up and clarify its highest attributes. Aristotle's three works on ethics: the Eudemian Ethics, Nicomachean Ethics, and the Magna Moralia are all closely related to politics rather than strictly individual ethics.
Happiness, or flourishing (eudemonia) for Aristotle is to be accomplished by building a rich inner life as opposed to the (perhaps later Stoic) ideal of simplifying one's needs and expectations. Aristotle asserts that man is by nature a social/political creature, and that one cannot flourish in isolation.
To distinguish which goods lead to happiness and which do not, Aristotle asks what is the particular nature of humanity, apart from all other creatures. The oft cited analogy is that of asking what is the nature of a knife apart from other kitchen utensils: it cuts. A good (or virtuous, Aristotle uses the same term, areté) knife cuts well. Humanity's special nature, that which sets us apart from other creatures is our employment of reason, our ability to act either rationally or irrationally.
'Ok,' you might think, 'it is our best nature to act rationally, so we ought to do that, right?' Well, strictly speaking, no. This is a case of the philosophical problem known by the fancy term: the naturalistic fallacy, aka. the is/ought problem. The problem is the reasoning that just because something is that it therefore should be that way. A simple example to illustrate this is to say, 'ethnic cleansing and perhaps soon genocide are again happening in Africa. We can say this 'is' a fact but certainly we would not say it 'ought to be' that way.' Some things that are, are wrong.
Aristotle gets around this by positing the virtues as the principles by which to judge whether something is good or bad. Reason is on the is side (of the is/ought divide). So reason itself cannot be employed to determine the rightness or wrongness of an action: you could have a very reasonable war criminal and a fairly irrational (but otherwise harmless) store clerk. What is needed as an logically external standard, and that is the virtues. They are external because they flow not from human nature per se, but from the practical nature of society and thus may vary from one society to the next.
~In the next post I hope to finish the exegesis on Aristotle, covering his discussion on the relationship between moral and intellectual virtues as well as his determination of what makes an act right.
Third, I will point out some of the continuity and difference in Kant's though. And finally I hope to finish with a comparison of some of these ideas in Aristotle to Buddhist ethics.