Aristotle, Nicomachean Ethics
H. Rackham, Ed.

[7] Were this not so, there would be no need for teachers of the arts, but everybody would be born a good or bad craftsman as the case might be. The same then is true of the virtues. It is by taking part in transactions with our fellow-men that some of us become just and others unjust; by acting in dangerous situations and forming a habit of fear or of confidence we become courageous or cowardly. And the same holds good of our dispositions with regard to the appetites, and anger; some men become temperate and gentle, others profligate and irascible, by actually comporting themselves in one way or the other in relation to those passions. In a word, our moral dispositions are formed as a result of the corresponding activities.