For Aristotle, before one examines the purpose of politics, one must understand the purposes of life.
In Nichomachean Ethics, Aristotle attempts to answer that question—what are the purposes, ends, or goals of human life.
He opens the book by noting that every activity or inquiry seems to aim at some goal or purpose. Any of these goals or purposes might also be called “goods.” These goods might include things like food, clothing, shelter, money, knowledge, and friendship. More importantly, however, he suggests that all these goods might be more than just goals or purposes or ends in themselves. They may also serve as means to some greater good. What could be this supreme good, purpose, or end?
As Aristotle puts it, “happiness more than anything else is thought to be just such an end because we always choose if for itself and never for any other reason.”
There are reasons we seek food, shelter, money, knowledge, friendship. We choose happiness, however, only for the sake of itself.
At this point, however, Aristotle throws his modern readers a twist. Usually we think of happiness as the psychological or emotional state that comes from acquiring whatever it is that we want. The Greek word translated happiness is eudaimonia, which means flourishing or thriving. It suggests the idea of getting not what we want, but what we need. While happiness in the emotional sense might accompany human flourishing, it is secondary.
Aristotle probes further into this question of happiness. What does it mean for a human to flourish? To find this answer, one must understand the function of a human being. And according to Aristotle, what distinguishes the functioning of human beings from every other creature is reason. Flourishing is living rationally. Therefore, Aristotle sees happiness as the rational activity of the soul.
Finally, he adds that any activity worth doing is worth doing well. “The function of a good man is to perform well and rightly . . . and if all this is so, the conclusion is that the good for man is an activity of the soul in accordance with excellence.”
Excellence derives from the Greek word arete, which most translations render virtue.
Aristotle's completed definition of happiness [flourishing] then is rational activity of the soul in accordance with virtue.
Or said another way, happiness, or the supreme goal of life, simply means to excel at being human.