On Nicomachean Ethics
Aristotle's Nicomachean Ethics is an admirable attempt to define the good life. It is empirical and insightful, and seeks to answer questions as relevant today as they were when it was written in the 4th century BC. It's also unfortunately extremely boring, at least in the two English translations I looked at (W.D. Ross and Joe Sachs). I don't particularly recommend reading it unless you have a strong interest in the history of ethics, as I think the Stoics or the Buddhists write much more practically and eloquently on the questions that concern Aristotle. On the other hand, I think he's an improvement over both deontological ethics and utilitarianism, so you may still find his ideas worth a look.
Aristotle makes many points with which I agree. He thinks that the good life (eudaimonia, sometimes badly translated as “happiness,” but better rendered as “flourishing”) is not a state but a continuous process, requiring constant practice not just to enact but even to understand. He believes that virtue consists in habits and principles, and not in isolated actions. He thinks this process is hard to attain to, and that most people will not reach it. He is an empiricist, as I am, and he bases much of his discourse on his keen observations of how humans actually behave. His ability to discern patterns in behaviour is probably his greatest strength. His description of the mean of moderation is particularly well-argued, and not as obvious as it sounded when I learned about it in high school.
However I disagree with him on many points. First, I think it is too simplistic to say that each action has a single end, and that the ultimate end of all actions is eudaimonia. It is useful to distinguish between intrinsic and extrinsic motivation, as he does to describe the relation of things like health and wealth to happiness, but I don't agree that people pursue health or wealth as intermediate ends, with the true end being eudaimonia.
I believe most actions are motivated by a variety of factors, which need not necessarily be known to those enacting them. People in general are not particularly good at knowing their own reasons for acting, and probably never (or virtually never) think things through in the kind of logical way that he prescribes. They do strive, of course, and give some thought to objectives, but I don't think it's common to consider why one strives.
Nor do I agree that such an exercise is necessary for morality. Aristotle argues that an action cannot be virtuous unless it is deliberate, with knowledge of what is being done, and further requires that the action is taken because it is a virtuous action. I think I more-or-less disagree with all three requirements. It might not make sense to have a virtuous action that is accidental, unknowing, and done for the wrong reasons, but I think it is possible to behave virtuously without any deliberation whatsoever, which seems to be opposed to Aristotle's view.
Basically I don't think deliberation necessarily leads to good actions. Aristotle often advocates the contemplation of the greatest good. If by this he means thinking rationally about the good, as he seems to, then I vehemently disagree that this has any effect on morality. If by contemplation Aristotle means meditation, or something along those lines, then I do think that's required for the cultivation of virtue. But meditation does not require reason, and may in some cases be opposed to it. (On the other hand, some rational understanding of the goals of meditation could be helpful—but I think this may vary. I've found that reason is useful for understanding the fruits of meditation, but it does not lead to their acquisition, and certainly not in the absence of practice.)
Aristotle seems to realise this on some level, saying that clever men can be wicked, but he continues to mandate reason as part of virtue. To me it seems clear that reason can be used to support any and all behaviour, virtuous and vicious alike. Nor are the right motivations required for an action to be good. It is perfectly possible to do the right action for the wrong reason, or, which is perhaps more likely, for no reason, i.e., with no altruism aforethought at all, which does not diminish its altruism. (In that sense it seems I'm a consequentialist, although I'm not quite arguing that consequences are the only thing that matter—just that premeditation is not a prerequisite for virtuous action.) I believe it is possible to behave morally without ever thinking about ethics in the systematic way that he advises.
Morality, in my view, basically requires meditation and practice. Virtue is not, as he suggests, a precondition either for contemplation/meditation or for the practice of virtue. You can meditate or cultivate the right actions without first being virtuous, and they will be effective even if you do them for some other reason than seeking virtue. In fact it is probably necessary to do some of this before virtue can arise at all. Good habits (ethos) are, as Aristotle argues, definitely required.
I might also agree that reason/contemplation is required for eudaimonia if he means that the use of the mind is a part of flourishing, just as the use of the body is. My dispute is not with his argument that contemplation is pleasurable and important; just that it is not a necessary condition for virtue.
I also think his way of dissecting the world into categories is probably counterproductive, because it risks dogmatism of the kind that his empiricism ought to avoid.
I dislike his attempt to make morals appear like mathematics. It is appealing, prima facie, to argue, as he does, that reasoning can be done about morals even if it can't be as precise as mathematics. But this line seems to imply there will be some bedrock premises on which the reasoning rests, when it is actually circular. E.g., he argues that a man cannot attain virtue without practical wisdom, but neither can he gain practical wisdom without virtue.
That's fine—there are many paradoxes in life. And perhaps he is observing that these things tend to arise in tandem. But he never explicitly acknowledges the many chicken-and-egg problems that he raises. In the end, I think focusing on the correct practices and ways of thinking (as the Stoics and Buddhists do) is more helpful than expecting a series of syllogisms to lead to the right actions.
I agree with him that practical wisdom (phronesis) should be distinguished from philosophic knowledge (sophia) and from technical knnowledge (techne). But the Buddhists are better empiricists and more practical on this point, explicitly explaining how to cultivate this kind of wisdom. The Buddha's claims seem more testable and less dogmatic than Aristotle's.
Overall I think he's a profound observer of human actions, with no small amount of insight. His emphasis on the contextual nature of most ethics is also welcome. But his extreme focus on reason is counterproductive, and at the end of the day the Ethics seem to me more descriptive of the problems of ethics than prescriptive of solutions thereto.