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 (John 9:41): "If you were blind, you should not have sin." Therefore blindness of mind is not a sin.
Obj. 2: Further, punishment differs from guilt. But blindness of mind is a punishment as appears from Isa. 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.
Obj. 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 First Part (Q. 84, AA. 7, 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 Obj. 1: The blindness that excuses from sin is that which arises from the natural defect of one who cannot see.
Reply Obj. 2: This argument considers the second kind of blindness which is a punishment.
Reply Obj. 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.