Summa Theologica, by St. Thomas Aquinas, , at sacred-texts.com
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 (FS, Q), 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:
(1) Whether blindness of mind is a sin?
(2) Whether dulness of sense is a sin distinct from blindness of mind?
(3) Whether these vices arise from sins of the flesh?
Objection 1: It would seem that blindness of mind is not a sin. Because, seemingly, that which excuses from sin is not itself a sin. Now blindness of mind excuses from sin; for it is written (Jn. 9:41): "If you were blind, you should not have sin." Therefore blindness of mind is not a sin.
Objection 2: Further, punishment differs from guilt. But blindness of mind is a punishment as appears from Is. 6:10, "Blind the heart of this people," for, since it is an evil, it could not be from God, were it not a punishment. Therefore blindness of mind is not a sin.
Objection 3: Further, every sin is voluntary, according to Augustine (De Vera Relig. xiv). Now blindness of mind is not voluntary, since, as Augustine says (Confess. x), "all love to know the resplendent truth," and as we read in Eccles. 11:7, "the light is sweet and it is delightful for the eyes to see the sun." Therefore blindness of mind is not a sin.
On the contrary, Gregory (Moral. xxxi, 45) reckons blindness of mind among the vices arising from lust.
I answer that, Just as bodily blindness is the privation of the principle of bodily sight, so blindness of mind is the privation of the principle of mental or intellectual sight. Now this has a threefold principle. One is the light of natural reason, which light, since it pertains to the species of the rational soul, is never forfeit from the soul, and yet, at times, it is prevented from exercising its proper act, through being hindered by the lower powers which the human intellect needs in order to understand, for instance in the case of imbeciles and madmen, as stated in the FP, Q, AA,8.
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. Hence it is written concerning some (Wis. 2:21): "Their own malice blinded them."
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. Sometimes it is due to the fact that a man's will is deliberately turned away from the consideration of that principle, according to Ps. 35:4, "He would not understand, that he might do well": whereas sometimes it is due to the mind being more busy about things which it loves more, so as to be hindered thereby from considering this principle, according to Ps. 57:9, "Fire," i.e. of concupiscence, "hath fallen on them and they shall not see the sun." In either of these ways blindness of mind is a sin.
Reply to Objection 1: The blindness that excuses from sin is that which arises from the natural defect of one who cannot see.
Reply to Objection 2: This argument considers the second kind of blindness which is a punishment.
Reply to Objection 3: To understand the truth is, in itself, beloved by all; and yet, accidentally it may be hateful to someone, in so far as a man is hindered thereby from having what he loves yet more.
Objection 1: It seems that dulness of sense is not a distinct sin from blindness of mind. Because one thing has one contrary. Now dulness is opposed to the gift of understanding, according to Gregory (Moral. ii, 49); and so is blindness of mind, since understanding denotes a principle of sight. Therefore dulness of sense is the same as blindness of mind.
Objection 2: Further, Gregory (Moral. xxxi, 45) in speaking of dulness describes it as "dullness of sense in respect of understanding." Now dulness of sense in respect of understanding seems to be the same as a defect in understanding, which pertains to blindness of mind. Therefore dulness of sense is the same as blindness of mind.
Objection 3: Further, if they differ at all, it seems to be chiefly in the fact that blindness of mind is voluntary, as stated above (A), while dulness of sense is a natural defect. But a natural defect is not a sin: so that, accordingly, dulness of sense would not be a sin, which is contrary to what Gregory says (Moral. xxxi, 45), where he reckons it among the sins arising from gluttony.
On the contrary, Different causes produce different effects. Now Gregory says (Moral. xxxi, 45) that dulness of sense arises from gluttony, and that blindness of mind arises from lust. Now these others are different vices. Therefore those are different vices also.
I answer that, Dull is opposed to sharp: and a thing is said to be sharp because it can pierce; so that a thing is called dull through being obtuse and unable to pierce. Now a bodily sense, by a kind of metaphor, is said to pierce the medium, in so far as it perceives its object from a distance or is able by penetration as it were to perceive the smallest details or the inmost parts of a thing. 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.
Now, by way of similitude to bodily sense, we speak of sense in connection with the intellect; and this latter sense is in respect of certain primals and extremes, as stated in Ethic. vi, even as the senses are cognizant of sensible objects as of certain principles of knowledge. Now this sense which is connected with understanding, does not perceive its object through a medium of corporeal distance, but through certain other media, as, for instance, when it perceives a thing's essence through a property thereof, and the cause through its effect. Consequently a man is said to have an acute sense in connection with his understanding, if, as soon as he apprehends a property or effect of a thing, he understands the nature or the thing itself, and if he can succeed in perceiving its slightest details: whereas a man is said to have a dull sense in connection with his understanding, if he cannot arrive at knowing the truth about a thing, without many explanations; in which case, moreover, he is unable to obtain a perfect perception of everything pertaining to the nature of that thing.
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.
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.
Objection 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.
Objection 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 to Objection 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 to Objection 2: The flesh acts on the intellective faculties, not by altering them, but by impeding their operation in the aforesaid manner.
Reply to Objection 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.