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 (Article 1), 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.
Source: Summa Theologica (New Advent)