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. ”
