He died in 322 B.C., at age 62. The Nicomachean Ethics was the first book ever written on ethics, and still one of the greatest and most influential. Aristotle assumes that all our actions aim at some end or good, that our ends form a hierarchy, and that there is one ultimate end. As happiness alone satisfies these conditions, happiness alone is our highest good. Happiness is our highest good.
Happiness is virtuous activity that fulfills our proper function.
Such activity is satisfying and brings pleasure. Physical pleasures, indeed, can tempt us with excesses and lead us away from virtue and happiness. The distinct human function that separates us from other beings is reason. The harpist's function is to play the harp and to play it well, and the heart's function is to pump blood and to pump it well. So also our function is to use reason and to use it well.