The doctrine of Aristotle (384-322 BC) that moral virtue can be defined as a disposition concerned with choice and lying in a mean.
Any given virtue lies between two extremes, for example courage lies in a mean between rashness and cowardice. The mean, however, is not an arithmetical mean, but is 'relative to us'; that is, to our natural tendencies.
Since we naturally tend more to cowardice than to rashness, the mean is nearer to rashness. (But sometimes Aristotle seems to say only that we should especially avoid our own pet vices.) The doctrine thus risks being vacuous, the mean being whatever point we ought to pursue; this being determined by the moral insight of the trained and practised 'man of practical wisdom'.
But Aristotle could say that the doctrine reminds us that there are always two opposite errors which we must avoid.
Whether all virtues can be so classified, without artificiality or triviality, may be disputed, but there is no mean of the mean or extremes: we don't have to avoid excess of virtue, nor (Aristotle's example) can a man commit adultery with the right woman at the right time in the right way.
Aristotle, Nicomachean Ethics, book 2, chs 5-9