Prudence (lat.:prudentia) is classically considered to be a virtue, and indeed, one of the Cardinal Virtues. The word comes from Old French prudence (13th century), from Latin prudentia foresight, sagacity, contraction of providentia foresight. It is often associated with Wisdom, Insight, and Knowledge. In this case, the virtue is the ability to judge between virtuous and vicious actions, not only in a general sense, but with regard to appropriate actions at a given time and place. Although prudence itself does not perform any actions, and is concerned solely with knowledge, all virtues had to be regulated by it. Distinguishing when acts are courageous, as opposed to reckless or cowardly, for instance, was an act of prudence. This is why it is classified as a cardinal which is to say pivotal virtue. Although prudence would be applied to any such judgment, the more difficult tasks, which distinguish a person as prudent, are those in which various goods have to be weighed against each other, as when a person is determining what would be best to give charitable donations, or how to punish a child so as to prevent repeating an offense.
Conventionally, prudence is the exercise of sound judgment in practical affairs.
In modern English, however, the word has become increasingly synonymous with cautiousness. In this sense, prudence names a reluctance to take risks, which remains a virtue with respect to unnecessary risks, but when unreasonably extended (i.e. over-cautiousness), can become the vice of cowardice.
In the Nichomachean Ethics, Aristotle gives a lengthy account of the virtue phronesis (Greek: ϕρονησιϛ), which has traditionally been translated as "prudence", although this has become increasingly problematic as the word has fallen out of common usage. More recently ϕρονησιϛ has been translated by such terms as "practical wisdom" or "practical judgment."
It is the cause in the sense that the virtues, which are defined to be the “perfected ability” of man as a spiritual person (spiritual personhood in the classical western understanding means having intelligence and free will), achieve their “perfection” only when they are founded upon prudence, that is to say upon the perfected ability to make right decisions. For instance, a person can live temperance when he has acquired the habit of deciding correctly the actions to take in response to his instinctual cravings.
Prudence is considered the measure of moral virtues since it provides a model of ethically good actions. “The work of art is true and real by its correspondence with the pattern of its prototype in the mind of the artist. In similar fashion, the free activity of man is good by its correspondence with the pattern of prudence.”(CITE) For instance, a stock broker using his experience and all the data available to him decides that it is beneficial to sell stock A at 2PM tomorrow and buy stock B today. The content of the decision (e.g., the stock, amount, time and means) is the product of an act of prudence, while the actual carrying out of the decision may involve other virtues like fortitude (doing it in spite of fear of failure) and justice (doing his job well out of justice to his company and his family). The actual act’s “goodness” is measured against that original decision made through prudence.
In Greek and Scholastic philosophy, “form” denotes that which provides a thing the specific characteristic that makes it what it is. With this language, prudence confers upon another virtues the form of its inner essence; that is, its specific character as a virtue. For instance, not all acts of telling the truth are considered good, considered as done with the virtue of honesty. What makes telling the truth a virtue is whether it is done with prudence. Telling a competitor the professional secrets of your company is not prudent and therefore not considered good and virtuous.
Judgments using reasons for evil ends or using evil means are considered to be made through “cunning” and “false prudence” and not through prudence.
For instance, in Just War theory, the government of a nation must weigh whether the harms they suffer are more than the harms that would be produced by their going to war against another nation that is harming them; the decision whether to go to war is therefore a prudential judgment.
In another case, a patient who has a terminal illness with no conventional treatment may hear of an experimental treatment. To decide whether to take it would require weighing on one hand, the cost, time, possible lack of benefit, and possible pain, disability, and hastened death, and on the other hand, the possible benefit and the benefit to others of what could be learned from his case.
Rules of Prudence are designed to serve self interest. "Do not drink the cleaning solution" would be a rule of prudence. This rule would be not considered a moral rule because while it is not morally wrong to drink cleaning solution, it does serve your best interest not to do so.