Nicomachean Ethics by Aristóteles — Read

We have thus found the genus to which virtue belongs; but we want to know, not only that it is a trained faculty, but also what species of trained faculty it is.

We may safely assert that the virtue or excellence of a thing causes that thing both to be itself in good condition and to perform its function well. The excellence of the eye, for instance, makes both the eye and its work good; for it is by the excellence of the eye that we see well. So the proper excellence of the horse makes a horse what he should be, and makes him good at running, and carrying his rider, and standing a charge.