/summa/FS/FSV/FS_Q80:
We must now consider the cause of sin, as regards the devil; and under this head there are four points of inquiry:
(1) Whether the devil is directly the cause of sin?
(2) Whether the devil induces us to sin, by persuading us inwardly?
(3) Whether he can make us sin of necessity?
(4) Whether all sins are due to the devil's suggestion?