I answer that, As stated above (Question [47], Article [2], ad 1; Article [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 (FS, Question [1], Article [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.