“… sin is nothing else than a bad human act. Now that an act is a human act is due to its being voluntary, 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. See Contra Faustum . xxii, 27
CHURCH FATHERS: Contra Faustum, Book XXII (Augustine), St. Thomas Aquinas, Summa Theologica, I, II, 71,6