We have been told that virtue comes about by choosing a mean between vicious extremes according to the right principle. This is only as helpful as telling a sick person that health comes about by choosing medicine according to what a doctor might prescribe. That is, we have no helpful understanding of virtue until we learn what this right principle is. To learn about the right principle, we must examine the intellectual virtues.
The soul is divided into a rational part and an irrational part. The rational part can be further divided into a contemplative part, which studies the invariable truths of science and mathematics, and a calculative part, which deals with the practical matters of human life. Right reasoning with respect to the contemplative intellect corresponds to truth. With the practical intellect, right reasoning corresponds to proper deliberation that leads to making the right choice.
There are five intellectual virtues by which the soul arrives at truth. First, scientific knowledge arrives at eternal truths by means of deduction or induction. Second, art or technical skill involves production according to proper reasoning. Third, prudence or practical wisdom helps us to pursue the good life generally. Fourth, intuition helps us to grasp first principles from which we derive scientific truths. Fifth, wisdom is a combination of scientific knowledge and intuition, which helps us arrive at the highest truths of all. Political science is a species of prudence, since it involves ensuring the good life for an entire city.
Resourcefulness, or good deliberation, is not the same thing as scientific knowledge, opinion, or conjecture. It is a process that helps achieve the ends envisaged by prudence. Understanding is a form of judgment regarding practical matters, which helps us determine what is equitable. Judgment, understanding, prudence, and intuition are all natural gifts that help us determine the right course of action.
The intellectual virtues help us to know what is just and admirable, and the moral virtues help us to do just and admirable deeds. We might wonder what value the intellectual virtues have, then, since knowledge is useless without action. First, the intellectual virtues lead to happiness, and so are ends in themselves. Second, the intellectual virtues help us determine the best means to the ends at which the moral virtues teach us to aim. Without prudence and cleverness, a well-disposed person can never be truly virtuous, because these intellectual virtues help us grasp the right principles of action.
At the beginning of Book II, Aristotle distinguishes between moral virtues, which we learn through habit and practice, and intellectual virtues, which we learn through instruction. Books II to V deal with the moral virtues. Book VI turns to intellectual virtues.