Objection 1. It would seem that prudence of the flesh is not a sin. For prudence is more excellent than the other moral virtues, since it governs them all. But no justice or temperance is sinful. Neither therefore is any prudence a sin.
Objection 2. Further, it is not a sin to act prudently for an end which it is lawful to love. But it is lawful to love the flesh, "for no man ever hated his own flesh" (Ephesians 5:29). Therefore prudence of the flesh is not a sin.
Objection 3. Further, just as man is tempted by the flesh, so too is he tempted by the world and the devil. But no prudence of the world, or of the devil is accounted a sin. Therefore neither should any prudence of the flesh be accounted among sins.
On the contrary, No man is an enemy to God save for wickedness according to Wisdom 14:9, "To God the wicked and his wickedness are hateful alike." Now it is written (Romans 8:7): "The prudence [Vulgate: 'wisdom'] of the flesh is an enemy to God." Therefore prudence of the flesh is a sin.
I answer that, As stated above (Question 47, Article 13), prudence regards things which are directed to the end of life as a whole. Hence prudence of the flesh signifies properly the prudence of a man who looks upon carnal goods as the last end of his life. Now it is evident that this is a sin, because it involves a disorder in man with respect to his last end, which does not consist in the goods of the body, as stated above (I-II, 02, 5). Therefore prudence of the flesh is a sin.
Reply to Objection 1. Justice and temperance include in their very nature that which ranks them among the virtues, viz. equality and the curbing of concupiscence; hence they are never taken in a bad sense. On the other hand prudence is so called from foreseeing [providendo], as stated above (47, 1; 49, 6), which can extend to evil things also. Therefore, although prudence is taken simply in a good sense, yet, if something be added, it may be taken in a bad sense: and it is thus that prudence of the flesh is said to be a sin.
Reply to Objection 2. The flesh is on account of the soul, as matter is on account of the form, and the instrument on account of the principal agent. Hence the flesh is loved lawfully, if it be directed to the good of the soul as its end. If, however, a man place his last end in a good of the flesh, his love will be inordinate and unlawful, and it is thus that the prudence of the flesh is directed to the love of the flesh.
Reply to Objection 3. The devil tempts us, not through the good of the appetible object, but by way of suggestion. Wherefore, since prudence implies direction to some appetible end, we do not speak of "prudence of the devil," as of a prudence directed to some evil end, which is the aspect under which the world and the flesh tempt us, in so far as worldly or carnal goods are proposed to our appetite. Hence we speak of "carnal" and again of "worldly" prudence, according to Luke 16:8, "The children of this world are more prudent [Douay: 'wiser'] in their generation," etc. The Apostle includes all in the "prudence of the flesh," because we covet the external things of the world on account of the flesh.
We may also reply that since prudence is in a certain sense called "wisdom," as stated above (47, 2, ad 1), we may distinguish a threefold prudence corresponding to the three kinds of temptation. Hence it is written (James 3:15) that there is a wisdom which is "earthly, sensual and devilish," as explained above (45, 1, ad 1), when we were treating of wisdom.
Source: Summa Theologica (New Advent)