Reply Obj. 1: Just as in the body the stronger the movement against the order of nature, the greater the weakness, so likewise, the stronger the movement of passion against the order of reason, the greater the weakness of the soul.
Reply Obj. 2: Sin consists chiefly in an act of the will, which is not hindered by weakness of the body: for he that is weak in body may have a will ready for action, and yet be hindered by a passion, as stated above (A. 1). Hence when we speak of sins of weakness, we refer to weakness of soul rather than of body. And yet even weakness of soul is called weakness of the flesh, in so far as it is owing to a condition of the flesh that the passions of the soul arise in us through the sensitive appetite being a power using a corporeal organ.
Reply Obj. 3: It is in the will's power to give or refuse its consent to what passion inclines us to do, and it is in this sense that our appetite is said to be under us; and yet this consent or dissent of the will is hindered in the way already explained (A. 1). ________________________
FOURTH ARTICLE [I-II, Q. 77, Art. 4]
Whether Self-love Is the Source of Every Sin?
Objection 1: It would seem that self-love is not the source of every sin. For that which is good and right in itself is not the proper cause of sin. Now love of self is a good and right thing in itself: wherefore man is commanded to love his neighbor as himself (Lev. 19:18). Therefore self-love cannot be the proper cause of sin.
Obj. 2: Further, the Apostle says (Rom. 7:8): "Sin taking occasion by the commandment wrought in me all manner of concupiscence"; on which words a gloss says that "the law is good, since by forbidding concupiscence, it forbids all evils," the reason for which is that concupiscence is the cause of every sin. Now concupiscence is a distinct passion from love, as stated above (Q. 3, A. 2; Q. 23, A. 4). Therefore self-love is not the cause of every sin.
Obj. 3: Further, Augustine in commenting on Ps. 79:17, "Things set on fire and dug down," says that "every sin is due either to love arousing us to undue ardor or to fear inducing false humility." Therefore self-love is not the only cause of sin.
Obj. 4: Further, as man sins at times through inordinate love of self, so does he sometimes through inordinate love of his neighbor. Therefore self-love is not the cause of every sin.
On the contrary, Augustine says (De Civ. Dei xiv, 28) that "self-love, amounting to contempt of God, builds up the city of Babylon." Now every sin makes man a citizen of Babylon. Therefore self-love is the cause of every sin.