Full Idea
Just saying that man's happiness is the supreme good seems a platitude, and some more distinctive account of it is still required. This might perhaps be achieved by grasping what is the function of man.
Gist of Idea
Perhaps we get a better account of happiness as the good for man if we know his function
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1097b22)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.75
A Reaction
Notice the 'perhaps', right at the heart of Aristotle's theory. The connection between happiness and function is not obvious. The connection is, of course, areté (virtue/excellence), which is known by the function, and generates the happiness.