Prudence is the cause of the other virtues’ being virtues at all. For example, there may be a kind of instinctive governance of instinctual cravings; but only prudence transforms this instinctive governance into the “virtue” of temperance. Virtue is a “perfected ability” of man as a spiritual person; and justice, fortitude, and temperance, as “abilities” of the whole man, achieve their “perfection” only when they are founded upon prudence, that is to say upon the perfected ability to make right decisions. Only by means of this perfected ability to make good choices are instinctive inclinations toward goodness exalted into the spiritual core of man’s decisions, from which truly human acts arise.
--Joseph Pieper