I answer that, Reason is the first principle of all human acts; and whatever other principles of human acts may be found, they obey reason somewhat, but in various ways. For some obey reason blindly and without any contradiction whatever: such are the limbs of the body, provided they be in a healthy condition, for as soon as reason commands, the hand or the foot proceeds to action. Hence the Philosopher says (Polit. i, 3) that "the soul rules the body like a despot," i.e. as a master rules his slave, who has no right to rebel. Accordingly some held that all the active principles in man are subordinate to reason in this way. If this were true, for man to act well it would suffice that his reason be perfect. Consequently, since virtue is a habit perfecting man in view of his doing good actions, it would follow that it is only in the reason, so that there would be none but intellectual virtues. This was the opinion of Socrates, who said "every virtue is a kind of prudence," as stated in Ethic. vi, 13. Hence he maintained that as long as man is in possession of knowledge, he cannot sin; and that every one who sins, does so through ignorance.
Now this is based on a false supposition. Because the appetitive faculty obeys the reason, not blindly, but with a certain power of opposition; wherefore the Philosopher says (Polit. i, 3) that "reason commands the appetitive faculty by a politic power," whereby a man rules over subjects that are free, having a certain right of opposition. Hence Augustine says on Ps. 118 (Serm. 8) that "sometimes we understand [what is right] while desire is slow, or follows not at all," in so far as the habits or passions of the appetitive faculty cause the use of reason to be impeded in some particular action. And in this way, there is some truth in the saying of Socrates that so long as a man is in possession of knowledge he does not sin: provided, however, that this knowledge is made to include the use of reason in this individual act of choice.
Accordingly for a man to do a good deed, it is requisite not only that his reason be well disposed by means of a habit of intellectual virtue; but also that his appetite be well disposed by means of a habit of moral virtue. And so moral differs from intellectual virtue, even as the appetite differs from the reason. Hence just as the appetite is the principle of human acts, in so far as it partakes of reason, so are moral habits to be considered virtues in so far as they are in conformity with reason.