Aristotle: Nicomachean Ethics
What kind of character state is virtue?
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Aristotle says that virtue is a state of character concerned with choice of a mean—more specifically, a choice of a mean between two extremes. This does not mean, of course, that all kinds of action admit of a spectrum; for there is no righteous mean for committing adultery, for example.
Aristotle, Nichomachean Ethics, Book II