Objection 1. It would seem that sin is unfittingly defined by saying: "Sin is a word, deed, or desire, contrary to the eternal law." Because "Word," "deed," and "desire" imply an act; whereas not every sin implies an act, as stated above (Article 5). Therefore this definition does not include every sin.
Objection 2. Further, Augustine says (De Duab. Anim. xii): "Sin is the will to retain or obtain what justice forbids." Now will is comprised under desire, in so far as desire denotes any act of the appetite. Therefore it was enough to say: "Sin is a desire contrary to the eternal law," nor was there need to add "word" or "deed."
Objection 3. Further, sin apparently consists properly in aversion from the end: because good and evil are measured chiefly with regard to the end as explained above (1, 3; k1 18, A4,6; 20, A2,3): wherefore Augustine (De Lib. Arb. i) defines sin in reference to the end, by saying that "sin is nothing else than to neglect eternal things, and seek after temporal things": and again he says (Qq. lxxxii, qu. 30) that "all human wickedness consists in using what we should enjoy, and in enjoying what we should use." Now the definition is question contains no mention of aversion from our due end: therefore it is an insufficient definition of sin.
Objection 4. Further, a thing is said to be forbidden, because it is contrary to the law. Now not all sins are evil through being forbidden, but some are forbidden because they are evil. Therefore sin in general should not be defined as being against the law of God.
Objection 5. Further, a sin denotes a bad human act, as was explained above (Article 1). Now man's evil is to be against reason, as Dionysius states (Div. Nom. iv). Therefore it would have been better to say that sin is against reason than to say that it is contrary to the eternal law.
On the contrary, the authority of Augustine suffices (Contra Faust. xxii, 27).
I answer that, As was shown above (Article 1), sin is nothing else than a bad human act. Now that an act is a human act is due to its being voluntary, as stated above (Question 1, Article 1), whether it be voluntary, as being elicited by the will, e.g. to will or to choose, or as being commanded by the will, e.g. the exterior actions of speech or operation. Again, a human act is evil through lacking conformity with its due measure: and conformity of measure in a thing depends on a rule, from which if that thing depart, it is incommensurate. Now there are two rules of the human will: one is proximate and homogeneous, viz. the human reason; the other is the first rule, viz. the eternal law, which is God's reason, so to speak. Accordingly Augustine (Contra Faust. xxii, 27) includes two things in the definition of sin; one, pertaining to the substance of a human act, and which is the matter, so to speak, of sin, when he says "word," "deed," or "desire"; the other, pertaining to the nature of evil, and which is the form, as it were, of sin, when he says, "contrary to the eternal law."
Reply to Objection 1. Affirmation and negation are reduced to one same genus: e.g. in Divine things, begotten and unbegotten are reduced to the genus "relation," as Augustine states (De Trin. v, 6,7): and so "word" and "deed" denote equally what is said and what is not said, what is done and what is not done.
Reply to Objection 2. The first cause of sin is in the will, which commands all voluntary acts, in which alone is sin to be found: and hence it is that Augustine sometimes defines sin in reference to the will alone. But since external acts also pertain to the substance of sin, through being evil of themselves, as stated, it was necessary in defining sin to include something referring to external action.
Reply to Objection 3. The eternal law first and foremost directs man to his end, and in consequence, makes man to be well disposed in regard to things which are directed to the end: hence when he says, "contrary to the eternal law," he includes aversion from the end and all other forms of inordinateness.
Reply to Objection 4. When it is said that not every sin is evil through being forbidden, this must be understood of prohibition by positive law. If, however, the prohibition be referred to the natural law, which is contained primarily in the eternal law, but secondarily in the natural code of the human reason, then every sin is evil through being prohibited: since it is contrary to natural law, precisely because it is inordinate.
Reply to Objection 5. The theologian considers sin chiefly as an offense against God; and the moral philosopher, as something contrary to reason. Hence Augustine defines sin with reference to its being "contrary to the eternal law," more fittingly than with reference to its being contrary to reason; the more so, as the eternal law directs us in many things that surpass human reason, e.g. in matters of faith.
Source: Summa Theologica (New Advent)