Sin makes the sinner stupid.
“St. Thomas Aquinas and Darkening of the Intellect
Saint Thomas Aquinas tells us why the intellect may become eclipsed. It’s due to sin. Not just any sin, but sin particularly related to intemperance. Intemperance is the inability to control the desire for pleasure.
Aquinas tells us that intemperance is the most disgraceful of sins,
“because [intemperance] is most repugnant to man’s clarity or beauty; inasmuch as the pleasures which are the matter of intemperance dim the light of reason from which all the clarity and beauty of virtue arises: wherefore these pleasures are described as being most slavish (emphasis added). (Summa Theologiae, II of II, Q. 142, Art. 4).
Interestingly, it is not just intemperance in general that causes the darkening of the intellect, but sexual sins in particular that tend to enslave our minds and souls. Aquinas says, for example,
“In the sin of fornication the soul is the body’s slave in a special sense, because at the moment of sinning it can think of nothing else: whereas the pleasure of gluttony, although carnal, does not so utterly absorb the reason…”(Summa Theologiae, I of II, Q. 72, Art. 4). ”