On Original Sin

The second does not suffice, because the will is moved of necessity by no object except by the last end alone.

3°. Therefore the devil alone can be for men a moral and indirect cause of sin¹, inasmuch as he proposes to the senses, whether internal or external, some appetible thing, stirs the sensitive appetite and the phantasy, and thence persuades reason that it is good and to be chosen. ¹ Ibid.

You will ask 2° how external sensible things become causes of sins.

I answer. External sensible things become causes of sins by moving the sensitive appetite. For when the sense is stirred by some object, it proposes it to the appetite, and the appetite allures reason; if reason fail from the consideration of the rule, it approves the delectable as such and offers it to the will; and if the will consent, it perfects the sin.

There follows concerning the cause of sin on the part of man, that is, concerning original sin. Wherefore let there be

St. Thomas, qq. 81, 82, 83.