Book 6: The Intellectual Virtues.
In order to ascertain what prudence is, we will first ask who they are whom we call prudent.
It seems to be characteristic of a prudent man that he is able to deliberate well about what is good or expedient for himself, not with a view to some particular end, such as health or strength, but with a view to well-being or living well.
This is confirmed by the fact that we apply the name sometimes to those who deliberate well in some particular field, when they calculate well the means to some particular good end, in matters that do not fall within the sphere of art. So we may say, generally, that a man who can deliberate well is prudent.
But no one deliberates about that which cannot be altered, nor about that which it is not in his power to do.
Now science, we saw, implies demonstration; but things whose principles or causes are variable do not admit of demonstration; for everything that depends upon these principles or causes is also variable; and, on the other hand, things that are necessarily determined do not admit of deliberation. It follows, therefore, that prudence cannot be either a science or an art: it cannot be a science, because the sphere of action is that which is alterable; it cannot be an art, because production is generically different from action.
It follows from all this that prudence is a formed faculty that apprehends truth by reasoning or calculation, and issues in action, in the domain of human good and ill; for while production has another end than itself, this is not so with action, since good action or well doing is itself the end.
For this reason Pericles and men who resemble him are considered prudent, because they are able to see what is good for themselves and for men; and this we take to be the character of those who are able to manage a household or a state.
This, too, is the reason why we call temperance σωϕροσύνη, signifying thereby that it is the virtue which preserves prudence. But what temperance preserves is this particular kind of judgment. For it is not any kind of judgment that is destroyed or perverted by the presentation of pleasant or painful objects (not such a judgment, for instance, as that the angles of a triangle are equal to two right angles), but only judgments about matters of practice. For the principles of practice [or the causes which originate action] are the ends for the sake of which acts are done; but when a man is corrupted by pleasure or pain, he straightway loses sight of the principle, and no longer sees that this is the end for the sake of which, and as a means to which, each particular act should be chosen and done; for vice is apt to obliterate the principle.
Our conclusion then is that prudence is a formed faculty which apprehends truth by reasoning or calculation, and issues in action, in the field of human good.
Moreover, art [or the artistic faculty] has its excellence [or perfect development] in something other than itself, but this is not so with prudence. Again, in the domain of art voluntary error is not so bad as involuntary, but it is worse in the case of prudence, as it is in the case of all the virtues or excellences. It is plain, then, that prudence is a virtue or excellence, and not an art.
And the rational parts of the soul or the intellectual faculties being two in number, prudence will be the virtue of the second, [the calculative part or] the faculty of opinion; for opinion deals with that which is variable, and so does prudence.
But it is something more than “a formed faculty of apprehending truth by reasoning or calculation;” as we see from the fact that such a faculty may be lost, but prudence, once acquired, can never be lost.