Aristotle was a philosopher in search of the chief good for human beings. This chief good is eudaimonia, which is often translated as 'happiness' (but can also be translated as 'thriving' or 'flourishing'). Aristotle sees pleasure, honour and virtue as significant 'wants' for people, and then argues that virtue is the most important of these.
In the Nicomachean Ethics, Aristotle makes the claim that happiness is something which is both precious and final. This seems to be so because it is a first principle or ultimate starting point. For, it is for the sake of happiness that we do everything else, and we regard the cause of all good things to be precious and divine. Moreover, since happiness is an activity of the soul in accordance with complete and perfect virtue, it is necessary to consider virtue, as this will be the best way of studying happiness.
How many of us today speak of happiness and virtue in the same breath? Aristotle's work in the Nicomachean Ethics is considered one of his greatest achievements, and by extension, one of the greatest pieces of philosophy from the ancient world. When the framers of the American Declaration of Independence were thinking of life, liberty, and the pursuit of happiness, there is little doubt they had an acquaintance with Aristotle's work connecting happiness, virtue, and ethics together.
When one thinks of ethical ideas such as an avoidance of extremes, of taking the tolerant or middle ground, or of taking all things in moderation, one is tapping into Aristotle's ideas. It is in the Nicomachean Ethics that Aristotle proposes the Doctrine of the Mean - he states that virtue is a 'mean state', that is, it aims for the mean or middle ground. However, Aristotle is often misquoted and misinterpreted here, for he very quickly in the text disallows the idea of the mean to be applied in all cases. There are things, actions and emotions, that do not allow the mean state. Thus, Aristotle tends to view virtue as a relative state, making the analogy with food - for some, two pounds of meat might be too much food, but for others, it might be too little. The mean exists between the state of deficiency, too little, and excessiveness, too much.
Aristotle proposes many different examples of virtues and vices, together with their mean states. With regard to money, being stingy and being illiberal with generosity are the extremes, the one deficient and the other excessive. The mean state here would be liberality and generosity, a willingness to buy and to give, but not to extremes. Anger, too, is highlighted as having a deficient state (too much passivity), an excessive state (too much passion) and a mean state (a gentleness but firmness with regard to emotions).
Aristotle states that one of the difficulties with leading a virtuous life is that it takes a person of science to find the mean between the extremes (or, in some cases, Aristotle uses the image of a circle, the scientist finding the centre). Many of us, being imperfect humans, err on one side or the other, choosing in Aristotle's words, the lesser of two evils. Aristotle's wording here, that a scientist is the only one fully capable of virtue, has a different meaning for scientist - this is a pre-modern, pre-Enlightenment view; for Aristotle, the person of science is one who is capable of observation and calculation, and this can take many different forms.
Aristotle uses different kinds of argumentation in the Nicomachean Ethics. He uses a dialectical method, as well as a functional method. In the dialectical method, there are opposing ideas held in tension, whose interactions against each other yield a result - this is often how the mean between extremes is derived. However, there are other times that Aristotle seems to prefer a more direct, functional approach. Both of these methods lead to the same understanding for Aristotle's sense of the rational - that humanity's highest or final good is happiness.
There is a discussion of the human soul (for this is where virtue and happiness reside). Aristotle argues that virtue is not a natural state; we are not born with nor do we acquire through any natural processes virtue, but rather through 'habitation', an embedding process or enculturation that makes these a part of our soul. However, it is not sufficient for Aristotle's virtue that one merely function as a virtuous person or that virtuous things be done. This is not a skill, but rather an art, and to be virtuous, one must live virtuously and act virtuously with intention as well as form.
Of course, one of the implications here is that virtue is a quantifiable thing, that periodically resurfaces in later philosophies. How do we calculate virtue?
This is a difficult question, and not one that Aristotle answers in any definitive way. However, more important than this is the key difference that Aristotle displayed setting himself apart from his tutor Plato; rather than seeing the possession of 'the good' or 'virtue' as the highest ideal, Aristotle is concerned with the practical aspects, the ethics of this. Based on Aristotle's lectures in Athens in the fourth century BCE, this remains one of the most important works on ethical and moral philosophy in history.