Summary

Portrait of Thomas Aquinas Thomas Aquinas,  Summa Theologica — Treatise on Habits…

“ On the contrary, Augustine proves (De Lib. Arb. i, 11) that "nothing else than his own will makes man's mind the slave of his desire." Now man does not become a slave to his desires, except through sin. Therefore the cause of sin cannot be the devil, but man's own will alone.
I answer that, Sin is an action: so that a thing can be directly the cause of sin, in the same way as anyone is directly the cause of an action; and this can only happen by moving that action's proper principle to act. Now the proper principle of a sinful action is the will, since every sin is voluntary.
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Source: Wikisource

Portrait of Thomas Aquinas Thomas Aquinas,  Summa Theologica — Treatise on Habits…

“ Further, a cause is more potent than its effect. But a habit produces its actions both as to their goodness and as to their badness. Therefore a habit is more potent than its act, both in goodness and in badness.
On the contrary, A man is justly punished for a vicious act; but not for a vicious habit, so long as no act ensues. Therefore a vicious action is worse than a vicious habit.
I answer that, A habit stands midway between power and act. Now it is evident that both in good and in evil, act precedes power, as stated in Metaph.
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Source: Wikisource

Portrait of Thomas Aquinas Thomas Aquinas,  Summa Theologica — Treatise on Habits…

“ Sin, in so far as it is inordinate, has the character of evil; but, in so far as it is an act, it has some good, at least apparent, for its end: so that, as an act, but not as being inordinate, it can be the cause, both final and efficient, of another sin. A sin has matter, not of which but about which it is: and it has its form from its end. ”
Source: Wikisource

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