Thomas Aquinas

“ 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thereto, nevertheless, on account of the pleasures of the body, it must needs happen that their attention is frequently withdrawn from this subtle contemplation: wherefore the unclean can know some truths, but their uncleanness is a clog on their knowledge. ”

“ 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. ”

“ Another principle of intellectual sight is a certain habitual light superadded to the natural light of reason, which light is sometimes forfeit from the soul. This privation is blindness, and is a punishment, in so far as the privation of the light of grace is a punishment. ”

“ 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. ”

“ A third principle of intellectual sight is an intelligible principle, through which a man understands other things; to which principle a man may attend or not attend. That he does not attend thereto happens in two ways. ”

“ Hence in corporeal things the senses are said to be acute when they can perceive a sensible object from afar, by sight, hearing, or scent, while on the other hand they are said to be dull, through being unable to perceive, except sensible objects that are near at hand, or of great power. ”

“ 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. ”