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."
Reply Obj. 1: Although some who are the slaves of carnal vices are at times capable of subtle considerations about intelligible things, on account of the perfection of their natural genius, or of some habit superadded thereto, nevertheless, on account of the pleasures of the body, it must needs happen that their attention is frequently withdrawn from this subtle contemplation: wherefore the unclean can know some truths, but their uncleanness is a clog on their knowledge.
Reply Obj. 2: The flesh acts on the intellective faculties, not by altering them, but by impeding their operation in the aforesaid manner.
Reply Obj. 3: It is owing to the fact that the carnal vices are further removed from the mind, that they distract the mind's attention to more remote things, so that they hinder the mind's contemplation all the more.