I answer that, The opinion of the Stoics, which Cicero adopts in the book on Paradoxes (Paradox. iii), was that all sins are equal: from which opinion arose the error of certain heretics, who not only hold all sins to be equal, but also maintain that all the pains of hell are equal. So far as can be gathered from the words of Cicero the Stoics arrived at their conclusion through looking at sin on the side of the privation only, in so far, to wit, as it is a departure from reason; wherefore considering simply that no privation admits of more or less, they held that all sins are equal. Yet, if we consider the matter carefully, we shall see that there are two kinds of privation. For there is a simple and pure privation, which consists, so to speak, in being corrupted; thus death is privation of life, and darkness is privation of light. Such like privations do not admit of more or less, because nothing remains of the opposite habit; hence a man is not less dead on the first day after his death, or on the third or fourth days, than after a year, when his corpse is already dissolved; and, in like manner, a house is no darker if the light be covered with several shades, than if it were covered by a single shade shutting out all the light. There is, however, another privation which is not simple, but retains something of the opposite habit; it consists in becoming corrupted rather than in being corrupted, like sickness which is a privation of the due commensuration of the humors, yet so that something remains of that commensuration, else the animal would cease to live: and the same applies to deformity and the like. Such privations admit of more or less on the part of what remains or the contrary habit. For it matters much in sickness or deformity, whether one departs more or less from the due commensuration of humors or members. The same applies to vices and sins: because in them the privation of the due commensuration of reason is such as not to destroy the order of reason altogether; else evil, if total, destroys itself, as stated in Ethic. iv, 5. For the substance of the act, or the affection of the agent could not remain, unless something remained of the order of reason. Therefore it matters much to the gravity of a sin whether one departs more or less from the rectitude of reason: and accordingly we must say that sins are not all equal.
Reply Obj. 1: To commit sin is unlawful on account of some inordinateness therein: wherefore those which contain a greater inordinateness are more unlawful, and consequently graver sins.
Reply Obj. 2: This argument looks upon sin as though it were a pure privation.
Reply Obj. 3: Virtues are proportionately equal in one and the same subject: yet one virtue surpasses another in excellence according to its species; and again, one man is more virtuous than another, in the same species of virtue, as stated above (Q. 66, AA. 1, 2). Moreover, even if virtues were equal, it would not follow that vices are equal, since virtues are connected, and vices or sins are not. ________________________
THIRD ARTICLE [I-II, Q. 73, Art. 3]
Whether the Gravity of Sins Varies According to Their Objects?
Objection 1: It would seem that the gravity of sins does not vary according to their objects. Because the gravity of a sin pertains to its mode or quality: whereas the object is the matter of the sin. Therefore the gravity of sins does not vary according to their various objects.
Obj. 2: Further, the gravity of a sin is the intensity of its malice. Now sin does not derive its malice from its proper object to which it turns, and which is some appetible good, but rather from that which it turns away from. Therefore the gravity of sins does not vary according to their various objects.
Obj. 3: Further, sins that have different objects are of different kinds. But things of different kinds cannot be compared with one another, as is proved in Phys. vii, text. 30, seqq. Therefore one sin is not graver than another by reason of the difference of objects.
On the contrary, Sins take their species from their objects, as was shown above (Q. 72, A. 1). But some sins are graver than others in respect of their species, as murder is graver than theft. Therefore the gravity of sins varies according to their objects.