Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I-II, q. 71, a. 6.

428. Sin taken theologically is defined by Augustine, bk. 22, c. Faustum, c. 27: Something said, done, or desired against the eternal law.

a) There is indicated that which is material in sin, namely an act which is distinguished as threefold to denote the threefold sin which can be committed in thought, word, and deed. Since however affirmation and negation are reduced to the same genus, under the name of something said, done, or desired (i.e. willed or loved) there also falls the sin of omission: not said, not done, not desired when one ought to say, do, or will. For the rest, the sin of omission radically consists in a positive act, because it is not voluntary unless it is derived from a deliberation by which man has resolved either to omit the precept or to do something incompatible with it.

b) It is said: against the law, so that it may be implicitly signified that for sin it is required that it be voluntary and free: for no one properly and formally acts against the law except through a voluntary and free act, or unless he wills freely to transgress it.

c) It is said: against the eternal law, because the eternal law is the first and essential rule of human acts, ordering them to the end, from which they have that they are good or evil; and although sins are also against reason, or even sometimes against human laws, these sins are not sins except because they are at the same time against the eternal law of God, from which all laws are derived and which commands obedience to them. This is the formal of sin, that it be against the law of God.

429. Sin strictly so called is also rightly defined philosophically: An act against right reason, or a human act incongruous with rational nature.

Thus, as S. Thomas notes, ad 5, « by theology sin is considered principally according as it is an offense against God, by the moral philosopher according as it is contradictory to reason ». Nevertheless there is no philosophical sin, i.e. repugnant to right reason, which is not at the same time theological, or contrary to the eternal law, because on the one hand whatever is against right reason is against the eternal law which is the reason of God, since right reason is nothing other than the participation of the eternal law in the created mind; on the other hand it is impossible that any man who has sufficient use of reason invincibly ignore God and the eternal law. The concept of philosophical sin is therefore not metaphysically repugnant: for an act sufficiently appears evil and a sin because it is repugnant to the rule of reason alone, abstracting (not excluding) from the eternal law; in fact, however, in the real order, it does not exist, as is evident from prop. 2 condemned as scandalous, rash, and erroneous by Alex. VIII¹.

430. Less well, sin is defined by some: A free transgression of the law of God, or a voluntary withdrawal from the divine rule.

Which definition, although true, is nevertheless incomplete because it is made according to the formal element² and does not indicate that the material element is of the essence of sin; it is also less proper than Augustine's definition: for sin is rather a voluntary act than a privation or withdrawal, because the will does not directly tend to a privation, but to a positive good, which has a privation attached to it; for good is its proper object. Whence in the first place and directly the positive element is to be posited.

431. Observation. — Just as the notion of sin, so also the definition does not fit perfectly except to actual mortal sin, imperfectly and in a qualified sense to others.