Accordingly dulness of sense in connection with understanding denotes a certain weakness of the mind as to the consideration of spiritual goods; while blindness of mind implies the complete privation of the knowledge of such things. Both are opposed to the gift of understanding, whereby a man knows spiritual goods by apprehending them, and has a subtle penetration of their inmost nature. This dulness has the character of sin, just as blindness of mind has, that is, in so far as it is voluntary, as evidenced in one who, owing to his affection for carnal things, dislikes or neglects the careful consideration of spiritual things.
This suffices for the Replies to the Objections. _______________________
THIRD ARTICLE [II-II, Q. 15, Art. 3]
Whether Blindness of Mind and Dulness of Sense Arise from Sins of the
Flesh?
Objection 1: It would seem that blindness of mind and dulness of sense do not arise from sins of the flesh. For Augustine (Retract. i, 4) retracts what he had said in his Soliloquies i, 1, "God Who didst wish none but the clean to know the truth," and says that one might reply that "many, even those who are unclean, know many truths." Now men become unclean chiefly by sins of the flesh. Therefore blindness of mind and dulness of sense are not caused by sins of the flesh.
Obj. 2: Further, blindness of mind and dulness of sense are defects in connection with the intellective part of the soul: whereas carnal sins pertain to the corruption of the flesh. But the flesh does not act on the soul, but rather the reverse. Therefore the sins of the flesh do not cause blindness of mind and dulness of sense.
Obj. 3: Further, all things are more passive to what is near them than to what is remote. Now spiritual vices are nearer the mind than carnal vices are. Therefore blindness of mind and dulness of sense are caused by spiritual rather than by carnal vices.
On the contrary, Gregory says (Moral. xxxi, 45) that dulness of sense arises from gluttony and blindness of mind from lust.
I answer that, The perfect intellectual operation in man consists in an abstraction from sensible phantasms, wherefore the more a man's intellect is freed from those phantasms, the more thoroughly will it be able to consider things intelligible, and to set in order all things sensible. Thus Anaxagoras stated that the intellect requires to be "detached" in order to command, and that the agent must have power over matter, in order to be able to move it. Now it is evident that pleasure fixes a man's attention on that which he takes pleasure in: wherefore the Philosopher says (Ethic. x, 4, 5) that we all do best that which we take pleasure in doing, while as to other things, we do them either not at all, or in a faint-hearted fashion.
Now carnal vices, namely gluttony and lust, are concerned with pleasures of touch in matters of food and sex; and these are the most impetuous of all pleasures of the body. For this reason these vices cause man's attention to be very firmly fixed on corporeal things, so that in consequence man's operation in regard to intelligible things is weakened, more, however, by lust than by gluttony, forasmuch as sexual pleasures are more vehement than those of the table. Wherefore lust gives rise to blindness of mind, which excludes almost entirely the knowledge of spiritual things, while dulness of sense arises from gluttony, which makes a man weak in regard to the same intelligible things. On the other hand, the contrary virtues, viz. abstinence and chastity, dispose man very much to the perfection of intellectual operation. Hence it is written (Dan. 1:17) that "to these children" on account of their abstinence and continency, "God gave knowledge and understanding in every book, and wisdom."