We must now consider the vices opposed to knowledge and understanding. Since, however, we have treated of ignorance which is opposed to knowledge, when we were discussing the causes of sins ([2421] FS, Q [76]), we must now inquire about blindness of mind and dulness of sense, which are opposed to the gift of understanding; and under this head there are three points of inquiry:

A1 Whether blindness of mind is a sin?

A2 Whether dulness of sense is a sin distinct from blindness of mind?

A3 Whether these vices arise from sins of the flesh?