Vices Opposed to Knowledge and Understanding

A person who turns away his mind from all consideration of God, or who so busies himself with creatural things that he has no time to think of God and of his own soul’s needs, is subject to mental and spiritual blindness; in so far as this is a person’s own fault, it is a sin.

Blindness of mind is a complete privation of the consideration of spiritual goods. Dullness of sense is a partial privation; it is a weakness, not a total absence, of mental vision which beholds spiritual goods. Thus dullness, in so far as it is voluntary, is also sinful.

It appears that both dullness of sense and blindness of mind arise primarily from sins of the flesh; the former from gluttony, and the latter from lust.