Objection 1. It would seem that prudence of the flesh is a mortal sin. For it is a mortal sin to rebel against the Divine law, since this implies contempt of God. Now "the prudence [Douay: 'wisdom'] of the flesh... is not subject to the law of God" (Romans 8:7). Therefore prudence of the flesh is a mortal sin.
Objection 2. Further, every sin against the Holy Ghost is a mortal sin. Now prudence of the flesh seems to be a sin against the Holy Ghost, for "it cannot be subject to the law of God" (Romans 8:7), and so it seems to be an unpardonable sin, which is proper to the sin against the Holy Ghost. Therefore prudence of the flesh is a mortal sin.
Objection 3. Further, the greatest evil is opposed to the greatest good, as stated in Ethic. viii, 10. Now prudence of the flesh is opposed to that prudence which is the chief of the moral virtues. Therefore prudence of the flesh is chief among mortal sins, so that it is itself a mortal sin.
On the contrary, That which diminishes a sin has not of itself the nature of a mortal sin. Now the thoughtful quest of things pertaining to the care of the flesh, which seems to pertain to carnal prudence, diminishes sin [Cf. Proverbs 6:30. Therefore prudence of the flesh has not of itself the nature of a mortal sin.
I answer that, As stated above (47, 2, ad 1; 13), a man is said to be prudent in two ways. First, simply, i.e. in relation to the end of life as a whole. Secondly, relatively, i.e. in relation to some particular end; thus a man is said to be prudent in business or something else of the kind. Accordingly if prudence of the flesh be taken as corresponding to prudence in its absolute signification, so that a man place the last end of his whole life in the care of the flesh, it is a mortal sin, because he turns away from God by so doing, since he cannot have several last ends, as stated above (I-II, 01, 5).
If, on the other hand, prudence of the flesh be taken as corresponding to particular prudence, it is a venial sin. For it happens sometimes that a man has an inordinate affection for some pleasure of the flesh, without turning away from God by a mortal sin; in which case he does not place the end of his whole life in carnal pleasure. To apply oneself to obtain this pleasure is a venial sin and pertains to prudence of the flesh. But if a man actually refers the care of the flesh to a good end, as when one is careful about one's food in order to sustain one's body, this is no longer prudence of the flesh, because then one uses the care of the flesh as a means to an end.
Reply to Objection 1. The Apostle is speaking of that carnal prudence whereby a man places the end of his whole life in the goods of the flesh, and this is a mortal sin.
Reply to Objection 2. Prudence of the flesh does not imply a sin against the Holy Ghost. For when it is stated that "it cannot be subject to the law of God," this does not mean that he who has prudence of the flesh, cannot be converted and submit to the law of God, but that carnal prudence itself cannot be subject to God's law, even as neither can injustice be just, nor heat cold, although that which is hot may become cold.
Reply to Objection 3. Every sin is opposed to prudence, just as prudence is shared by every virtue. But it does not follow that every sin opposed to prudence is most grave, but only when it is opposed to prudence in some very grave matter.
Source: Summa Theologica (New Advent)