In the Nicomachaean Ethics, Aristotle writes that happiness “is not a disposition” and “we must rather class happiness as an activity.” This doubtless sounds odd to many people, but the word has many meanings, and we’re looking at a translation (in this case, by W. D. Ross) from the Greek. The original word was probably “eudaemonia,” for which “happiness” is only a rough equivalent. “Good living” might be more accurate. Further on he writes that “the life according to reason is best and pleasantest, since reason more than anything else is man. This life is therefore also the happiest.”
He is careful to distinguish happiness from pleasure and amusement. “The happy life is thought to be virtuous; now a virtuous life requires exertion, and does not consist in amusement.” Specifically, it’s the exertion of the mind, because our reasoning capacity is the most important thing about us. It’s desirable in itself, not for the sake of some further goal.
This is an attractive thought, that the process understanding is the best thing. At the same time, there’s something passive about it. The best thing for Aristotle is the contemplative life, the life of the philosopher. He thought he had the best job in the world, and that’s not a bad thing. But it lacks something in engagement with the world. Reason may be our most distinctive characteristic, but we’re beings of both body and mind.
Aristotle lived in a time when people hadn’t fully developed the idea of reason as a means to improving the human condition. Archimedes lived about a century later and was one of the people who advanced the use of reason for practical purposes. The important point which Aristotle made is that happiness (or the good life) comes from thinking, not from pursuing physical pleasures, and that it’s active, not passive. It’s necessary to understand this much before discovering all its practical applications.