Summa Theologiae Moralis, vol. I: De Principiis

410. Object of the Treatise. — Moral theology treats of human acts, by which we attain the end, happiness. Those are acts which are conformed to the dictate of right conscience and of the eternal law. But there are also human acts which do not conform to the law of God or to the dictate of right conscience, by which we do not attain the end, and which are called sins. Whence after the treatises on the end, on human acts, and on their rules, namely conscience and law, the consideration of evil acts or sins rightly comes.

411. The Treatise is divided into 3 parts:

Part 1 treats of sin considered in itself:

1. of the essence of sin; — 2) of the distinction of sins a) specific, b) numeric; — 3) of the comparison and gravity of sins; — 4) of the subject of sin, where internal sins are treated.

Part 2 treats of the causes of sins:

1. of interior causes: a) of ignorance on the part of reason, b) of passion on the part of the sensitive appetite, c) of malice on the part of the will;

2. of exterior causes: a) of God permitting, b) of the devil tempting, c) of man cooperating;

3. of the sin which is the cause of another sin, namely of the manner in which one sin causes another, and of the capital sins.

Part 3 treats of the effects of sin:

1. of the diminution of the good of nature; — 2) of the stain or guilt of sin; — 3) of the liability to punishment; — 4) of mortal and venial sin, which are distinguished according to their diverse effects: a) of the distinction of mortal from venial sin, b) of its gravity, c) of the conditions required for it, d) of the division of mortal and venial sins, e) of their conversion, f) of venial sin in itself.

412. Importance of the Treatise. — The treatise is of the greatest importance, because it applies the general abstract principles about human acts more determinately and concretely to sins, which are to be avoided by everyone above all, and are to be esteemed and judged by the priest and confessor in others¹.