To that, however, which is objected, that the flesh is in our power; it must be said, that even if it is in our power according to the order of nature, yet on according to the perversity of concupiscence in many it both reigns as handmaid and serves as lady, and too much love for the flesh makes the belly be man’s god; and for that reason it is not in our power, until we are liberated from the power of this body, with the help of the grace of God through Christ Jesus Our Lord. For so great is the force of love, that it subjects in a certain manner the lover to the beloved. Wherefore even if a son is in the power of (his) father, yet it is not easy for the father to slay or afflict the son, but (rather) very difficult; so must it be understood in the proposed, because no one ever holds his own flesh in hatred, but he nourishes and keeps it warm.
St. Bonaventure, Commentary on the Sentences of Peter Lombard, 2d21a2q3ad4. (The question is whether temptations of the flesh are more difficult to overcome than temptations of the devil; Bonaventure's response is that if we are considering the same sin, temptation of the flesh is the more difficult temptation.)