11 But some one will say to me that it is one thing to live after man, another thing to live after the flesh; because man forsooth is a rational creature, and there is in him a rational soul, whereby he differs from the beast: but the flesh is the lowest and earthly part of man, and thus to live after it is faulty: and for this reason, he who lives after man, assuredly lives not after the flesh, but rather after that part of man, whereby he is man, that is, after the spirit of the mind whereby he excels the beasts.
But this discussion is perhaps of some force in the schools of philosophers: but we, in order to understand the Apostle of Christ, ought to observe in what manner the Christian books are used to speak; at any rate it is the belief of all of us, to whom to live is Christ, that Man was taken unto Himself by the Word of God, not surely without a rational soul, as certain heretics will have it; and yet we read, “The Word was made flesh.” What is to be here understood by “flesh,” but Man?
“And all flesh shall see the salvation of God.” What can be understood, but all men? “Unto You shall all flesh come.” What is it, but all men? “You have given unto Him power over all flesh.” What is it, but all men? “Of the works of the Law shall no flesh be justified.” What is it, but no man shall be justified? And this the same Apostle in another place confessing more plainly says, “Man shall not be justified of the works of the Law.” The Corinthians also he rebukes, saying, “Are ye not carnal, and walk after man?” After he had called them carnal, he says not, you walk after the flesh, but after man, forasmuch as by this also what would he have understood, but after the flesh?
For surely if to walk, that is, to live, after the flesh deserved blame, but after man deserved praise, he would not say by way of rebuke, “ye walk after man.” Let man recognize the reproach; let him change his purpose, let him shun destruction. Hear you man: walk not after man, but after Him Who made man. Fall not away from Him Who made you, even unto yourself. For a man said, who yet lived not after man, “Not that we are sufficient to think any thing from ourselves, as though of ourselves: but our sufficiency is of God.” Consider if he lived after man, who spoke these things with truth.
Therefore the Apostle, admonishing man not to live after man, restores man to God. But whoso lives not after man, but after God, assuredly lives not even after himself, because himself also is a man. But he is therefore said also to live after the flesh, when he so lives; because also when the flesh alone has been named, man is understood, as we have already shown: just as when the soul alone has been named, man is understood: whence it is said, “Let every soul be subject unto the higher powers,” that is, every man; and, “Seventy-five souls went down into Egypt with Jacob,” that is, seventy-five men.
Therefore live not after yourself, O man: you had thence perished, but you were sought. Live not then, I say, after yourself, O man; you had thence perished, but you were found. Accuse not the nature of the flesh, when you hear it said, “If you shall live after the flesh, you shall die.” For thus could it be said, and most truly could it, If you shall live after yourselves you shall die. For the devil has not flesh, and yet, because he would live after himself, “he abode not in the truth.” What wonder therefore, if, living after himself, “when he speaks a lie, he speaks of his own,” which the Truth spoke truly of him.
Source: On Continence (New Advent)