...[when a thing does] whatever is appropriate to it according to its nature, it acts of itself; but when it is moved by something exterior, it does not act of itself, but by the influence of that other: and this is a kind of slavery. now according to his nature, man is rational. And thus when he acts according to reason, he is acting by his own proper motion and is acting of himself; and this is a characteristic of freedom. But when he sins, he is acting outside reason; and then he is moved by another, being held back by the limitations imposed by that other. Therefore, everyone who commits sin is a slave of sin. "Whatever overcomes a person, is that to which he is a slave" (2 Pet. 2:9).
[St. Thomas Aquinas, Commentary on the Gospel of John, Chapters 6-12, Larcher, tr., CUA Press (Washington DC: 2010), p. 126.]