Prudence, prudentia, from the Greek phronesis, is the virtue of practical reason: the capacity to judge rightly what should be done in a particular case, and it is called by Aquinas the charioteer of the virtues, since none of the others can act well without it. It is not caution, and the modern narrowing of the word to mean carefulness has obscured what the tradition meant. Aristotle treats phronesis in the Nicomachean Ethics as the intellectual virtue that governs the moral ones, distinguishing it from sophia, theoretical wisdom, and holding that it cannot be possessed without moral virtue nor moral virtue without it. Aquinas takes over the analysis and gives prudence the parts of memory, understanding, docility, shrewdness, reason, foresight, circumspection and caution, which together describe how a good judgment is actually reached.
Aquinas, in the Summa Theologiae, further analyzes prudence's single act into three internal moments, taking counsel (consilium), judging correctly among the options considered (iudicium), and commanding the will to act on that judgment (praeceptum), holding that a defect at any of the three stages spoils the whole. He also distinguishes true prudence carefully from what he calls astutia, a crafty cleverness that reasons skillfully toward a bad end, insisting that prudence properly speaking cannot exist apart from a will already ordered toward the genuine good, so that skill in practical reasoning alone, without right desire, counterfeits the virtue rather than possessing it.