Aristotle: Nicomachean Ethics
What warning from Book I does Aristotle issue again in Book II when dealing with the question of prescribing moral acts?
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Aristotle warns his reader in Book II that we should not expect the kind of precision in ethics that we have in other sciences such as mathematics. This is not a defect of the discipline of ethics, of course, it is simply the nature of the discipline. This is exactly what Aristotle said earlier in Book I.
Aristotle, Nichomachean Ethics, Book II