II-II Q. XLVI. ART. II.
Article II.–Is stupidity a sin?
R. Stupidity implies a dulness of perception in judging, particularly about the Highest Cause, the Last End and Sovereign Good. This may come of natural incapacity, and that is not a sin. Or it may come of man burying his mind so deep in earthly things as to render his perceptions unfit to grasp the things of God, according to the text: “The sensual man perceiveth not these things that are of the Spirit of God;” and such stupidity is a sin.
2. Though no one wishes to be stupid, still people do wish for what leads to stupidity, by withdrawing their thoughts from things spiritual and burying them in things of the earth. So it is also with other sins; for the lustful man wants the pleasure to which the sin is attached, though he does not absolutely wish for the sin; for he would like to enjoy the pleasure without the sin.