Man's virtue is the perfection of his nature. By the proper nature of his virtue, every being becomes perfect, and arrives at the climax of its excellence. Thus the virtue of the horse is that which makes the best of the horse's nature. The same reasoning applies to details. Thus the virtue of the eyes is acuteness of vision; and this is the climax of the eyes' nature. The virtue of the ears is acuteness of hearing; and this is the [aural] nature's climax. The virtue of the feet is swiftness; and this is the pedal nature's climax.
Every virtue, however, should include these three things: reason, power, and deliberate choice. Reason indeed, judges and contemplates; power prohibits and vanquishes; and deliberate choice loves and enjoys propriety. Therefore to judge and contemplate pertain to the intellectual part of the soul; to prohibit and vanquish are the peculiarity of the irrational part of the soul; and to love and enjoy propriety includes both rational and irrational parts of the soul; for deliberate choice consists of the discursive energy of reason, and appetite. Intention therefore, pertains to the rational, but appetite to the irrational parts of the soul.
We may discern the multitude of the virtues by observing the parts of the soul; also the growth and nature of virtue. Of the soul's parts, two rank first: the rational and the irrational. It is by their rational that we judge and contemplate; by the irrational we are impelled and desire. These are either consonant or discordant, their strife and dissonance being produced be excess or defect. The rational part's victory over the irrational produces endurance and continence. When the rational leads, the irrational follows, both accord, and produce virtue. That is why endurance and continence are generally accompanied by pain; for endurance resists pain and continence pleasure. However, incontinence and effeminacy neither' resist nor vanquish pleasure. That is why men fly from good through pain, but reject it through pleasure. Likewise praise and blame, and everything beautiful in human conduct, are produced in these parts of the soul. This explains the nature, of virtue.
Let us study virtue's kinds and parts. Since the soul is divided into two parts, the rational and the irrational, the latter is also divided into two, the irascible and appetitive part. By the rational we judge and contemplate; by the irrational we are impelled and desire. The irascible part defends us, and revenges incidental molestations; the appetitive directs and preserves the body's proper constitution. So we see that the numerous virtues with all their differences and peculiarities do little more than conform to the distinctive parts of the soul.