Aristotle's virtue ethics in the Nicomachean Ethics explains:
- (A) Correct: The function (ergon) argument – we understand virtue by knowing the proper function of a thing, e.g., the function of a human is rational activity, and virtue is what makes this function excellent.
- (B) Correct: Virtue involves practical wisdom (phronesis) and hence deliberation. Virtue is not merely a feeling but a rational disposition to choose well.
- (C) Incorrect: Virtue is acquired by habit and practice, not by mere theoretical knowledge.
- (D) Correct: Virtue is often described as the mean between two extremes (the doctrine of the mean) — e.g., courage lies between recklessness and cowardice.
\[
\boxed{\text{Therefore, A, B, and D are correct.}}
\]