linguistic:translation, quality:corruption, structural:reformatting, linguistic:voice_change, structural:summarization
Sin

By repeating a good action, man acquires a moral habit or a quality that enables him to do the good gladly and easily. This is true only of the intellectual and moral virtues (which St. Thomas treats after the manner of Aristotle); the theological virtues are imparted by God to man as a "disposition", from which the acts here proceed; while they strengthen, they do not form it. The "disposition" of evil is the opposite alternative.

An act becomes evil through deviation from the reason and from divine moral law. Therefore, sin involves two factors:

Sin has its origin in the will, which decides (against reason) for a "changeable good". Since the will also moves the other powers of man, sin has its seat in these too. By choosing such a lower good as its end, the will is misled by self-love, so that this works as cause in every sin. God is not the cause of sin since, on the contrary, he draws all things to himself; but from another side, God is the cause of all things, so he is efficacious also in sin as actio but not as ens. The devil is not directly the cause of sin, but he incites the imagination and the sensuous impulse of man (as men or things may also do).

Sin is original sin. Adam's first sin passes through himself to all the succeeding race; because he is the head of the human race and "by virtue of procreation human nature is transmitted and along with nature its infection." The powers of generation are, therefore, designated especially as "infected". The thought is involved here by the fact that St. Thomas, like other scholastics, believed in creationism; he therefore taught that souls are created by God.

Two things, according to St. Thomas, constituted man's righteousness in paradise:

Both are lost through original sin, which, in form, is the "loss of original righteousness". The consequence of this loss is the disorder and maiming of man's nature, which shows itself in "ignorance; malice, moral weakness, and especially in concupiscentia, which is the material principle of original sin." The course of thought here is as follows: when the first man transgressed the order of his nature appointed by nature and grace, he (and with him the human race) lost this order. This negative state is the essence of original sin. From it follow an impairment and perversion of human nature in which thenceforth lower aims rule, contrary to nature, and release the lower element in man.

Since sin is contrary to the divine order, it is guilt and subject to punishment. Guilt and punishment correspond to each other; and since the "apostasy from the invariable good which is infinite," fulfilled by man, is unending, it merits everlasting punishment.

God works even in sinners to draw them to the end by "instructing through the law and aiding by grace." The law is the "precept of the practical reason". As the moral law of nature, it is the participation of the reason in the all-determining "eternal reason"; but since man falls short in his appropriation of this law of reason, there is need of a "divine law"; and since the law applies to many complicated relations, the practicae dispositiones of the human law must be laid down.