Aristotle on Moral Virtues

Again, the causes or means that bring about any form of excellence are the same as those that destroy it, and similarly with art; for it is as a result of playing the harp that people become good and bad harpists. The same principle applies to builders and all other craftsmen. Men will become good builders as a result of building well, and bad ones as a result of building badly. Otherwise there would be no need of anyone to teach them: they would all be born either good or bad. Now this holds good also of the virtues. It is the way that we behave in our dealings with other people which makes us just or unjust, and the way that we behave in the face of danger, accustoming ourselves to be timid or confident, that makes us brave or cowardly. Similarly in situations involving desires and angry feelings: some people are temperate and patient from one kind of conduct in such situations, others licentious and choleric from another. In a word, then, like activities produce like dispositions. Hence we must give our activities a certain quality, because it is their characteristics that determine the resulting dispositions. So it is a matter of no little importance what sort of habits we form from the earliest age - it makes a vast difference, or rather all the difference in the world.

(From the Ethics of Aristotle)