Aristotle (384-322 BCE)
“Virtue is a purposive disposition, lying in a mean that is relative to us and determined by a rational principle, and by that which a prudent man would use to determine it. It is a mean between two kinds of vice, one of excess and the other of deficiency.”
- From The Nicomachean Ethics (Bekker page 1107a) by Aristotle, translated by J. A. K. Thomson (Penguin Classics, 2004).