Objection 1. It would seem that prudence is not a virtue necessary to lead a good life. For as art is to things that are made, of which it is the right reason, so is prudence to things that are done, in respect of which we judge of a man's life: for prudence is the right reason about these things, as stated in Ethic. vi, 5. Now art is not necessary in things that are made, save in order that they be made, but not after they have been made. Neither, therefore is prudence necessary to man in order to lead a good life, after he has become virtuous; but perhaps only in order that he may become virtuous.
Objection 2. Further, "It is by prudence that we are of good counsel," as stated in Ethic. vi, 5. But man can act not only from his own, but also from another's good counsel. Therefore man does not need prudence in order to lead a good life, but it is enough that he follow the counsels of prudent men.
Objection 3. Further, an intellectual virtue is one by which one always tells the truth, and never a falsehood. But this does not seem to be the case with prudence: for it is not human never to err in taking counsel about what is to be done; since human actions are about things that may be otherwise than they are. Hence it is written (Wisdom 9:14): "The thoughts of mortal men are fearful, and our counsels uncertain." Therefore it seems that prudence should not be reckoned an intellectual virtue.
On the contrary, It is reckoned with other virtues necessary for human life, when it is written (Wisdom 8:7) of Divine Wisdom: "She teacheth temperance and prudence and justice and fortitude, which are such things as men can have nothing more profitable in life."
I answer that, Prudence is a virtue most necessary for human life. For a good life consists in good deeds. Now in order to do good deeds, it matters not only what a man does, but also how he does it; to wit, that he do it from right choice and not merely from impulse or passion. And, since choice is about things in reference to the end, rectitude of choice requires two things: namely, the due end, and something suitably ordained to that due end. Now man is suitably directed to his due end by a virtue which perfects the soul in the appetitive part, the object of which is the good and the end. And to that which is suitably ordained to the due end man needs to be rightly disposed by a habit in his reason, because counsel and choice, which are about things ordained to the end, are acts of the reason. Consequently an intellectual virtue is needed in the reason, to perfect the reason, and make it suitably affected towards things ordained to the end; and this virtue is prudence. Consequently prudence is a virtue necessary to lead a good life.
Reply to Objection 1. The good of an art is to be found, not in the craftsman, but in the product of the art, since art is right reason about things to be made: for since the making of a thing passes into external matter, it is a perfection not of the maker, but of the thing made, even as movement is the act of the thing moved: and art is concerned with the making of things. On the other hand, the good of prudence is in the active principle, whose activity is its perfection: for prudence is right reason about things to be done, as stated above (Article 4). Consequently art does not require of the craftsman that his act be a good act, but that his work be good. Rather would it be necessary for the thing made to act well (e.g. that a knife should carve well, or that a saw should cut well), if it were proper to such things to act, rather than to be acted on, because they have not dominion over their actions. Wherefore the craftsman needs art, not that he may live well, but that he may produce a good work of art, and have it in good keeping: whereas prudence is necessary to man, that he may lead a good life, and not merely that he may be a good man.
Reply to Objection 2. When a man does a good deed, not of his own counsel, but moved by that of another, his deed is not yet quite perfect, as regards his reason in directing him and his appetite in moving him. Wherefore, if he do a good deed, he does not do well simply; and yet this is required in order that he may lead a good life.
Reply to Objection 3. As stated in Ethic. vi, 2, truth is not the same for the practical as for the speculative intellect. Because the truth of the speculative intellect depends on conformity between the intellect and the thing. And since the intellect cannot be infallibly in conformity with things in contingent matters, but only in necessary matters, therefore no speculative habit about contingent things is an intellectual virtue, but only such as is about necessary things. On the other hand, the truth of the practical intellect depends on conformity with right appetite. This conformity has no place in necessary matters, which are not affected by the human will; but only in contingent matters which can be effected by us, whether they be matters of interior action, or the products of external work. Hence it is only about contingent matters that an intellectual virtue is assigned to the practical intellect, viz. art, as regards things to be made, and prudence, as regards things to be done.
Source: Summa Theologica (New Advent)