I answer that, As stated above (Q. 75, A. 1), the proper and direct cause of sin is to be considered on the part of the adherence to a mutable good; in which respect every sinful act proceeds from inordinate desire for some temporal good. Now the fact that anyone desires a temporal good inordinately, is due to the fact that he loves himself inordinately; for to wish anyone some good is to love him. Therefore it is evident that inordinate love of self is the cause of every sin.
Reply Obj. 1: Well ordered self-love, whereby man desires a fitting good for himself, is right and natural; but it is inordinate self-love, leading to contempt of God, that Augustine (De Civ. Dei xiv, 28) reckons to be the cause of sin.
Reply Obj. 2: Concupiscence, whereby a man desires good for himself, is reduced to self-love as to its cause, as stated.
Reply Obj. 3: Man is said to love both the good he desires for himself, and himself to whom he desires it. Love, in so far as it is directed to the object of desire (e.g. a man is said to love wine or money) admits, as its cause, fear which pertains to avoidance of evil: for every sin arises either from inordinate desire for some good, or from inordinate avoidance of some evil. But each of these is reduced to self-love, since it is through loving himself that man either desires good things, or avoids evil things.
Reply Obj. 4: A friend is like another self (Ethic. ix): wherefore the sin which is committed through love for a friend, seems to be committed through self-love. ________________________
FIFTH ARTICLE [I-II, Q. 77, Art. 5]
Whether Concupiscence of the Flesh, Concupiscence of the Eyes, and
Pride of Life Are Fittingly Described As Causes of Sin?
Objection 1: It would seem that "concupiscence of the flesh, concupiscence of the eyes, and pride of life" are unfittingly described as causes of sin. Because, according to the Apostle (1 Tim. 6:10), "covetousness [*Douay: 'The desire of money'] is the root of all evils." Now pride of life is not included in covetousness. Therefore it should not be reckoned among the causes of sin.
Obj. 2: Further, concupiscence of the flesh is aroused chiefly by what is seen by the eyes, according to Dan. 13:56: "Beauty hath deceived thee." Therefore concupiscence of the eyes should not be condivided with concupiscence of the flesh.
Obj. 3: Further, concupiscence is desire for pleasure, as stated above (Q. 30, A. 2). Now objects of pleasure are perceived not only by the sight, but also by the other senses. Therefore "concupiscence of the hearing" and of the other senses should also have been mentioned.