The rebellion of the aggressive and sensual parts against reason cannot be quelled entirely by means of virtue; for since through their own nature those parts aim at what is good according to the senses, they sometimes conflict with reason. It could possibly happen, though, by means of divine power, which can even change something's nature. In any case, the rebellion is reduced through virtue, insofar as the powers in question become accustomed to obeying reason. Then they have what they need for virtue, from something outside, i.e. from the rule of reason over them. In themselves, however, they retain something of their own movements, which are sometimes contrary to reason.
Thomas Aquinas, Disputed Questions on the Virtues (q. 1, a. 4 ad 7), Atkins and Williams, ed. CUP (New York: 2005) p. 25.
