Reply Obj. 1:  Non-being cannot be the direct cause of anything:  but it can be an accidental cause, as being the removal of an impediment.

Reply Obj. 2:  As knowledge, which is removed by ignorance, regards sin as turning towards something, so too, ignorance of this respect of a sin is the cause of that sin, as removing its impediment.

Reply Obj. 3:  The will cannot turn to that which is absolutely unknown:  but if something be known in one respect, and unknown in another, the will can will it.  It is thus that ignorance is the cause of sin:  for instance, when a man knows that what he is killing is a man, but not that it is his own father; or when one knows that a certain act is pleasurable, but not that it is a sin. ________________________

SECOND ARTICLE [I-II, Q. 76, Art. 2]

Whether Ignorance Is a Sin?

Objection 1:  It would seem that ignorance is not a sin.  For sin is “a word, deed or desire contrary to God’s law,” as stated above (Q. 71, A. 5).  Now ignorance does not denote an act, either internal or external.  Therefore ignorance is not a sin.

Obj. 2:  Further, sin is more directly opposed to grace than to knowledge.  Now privation of grace is not a sin, but a punishment resulting from sin.  Therefore ignorance which is privation of knowledge is not a sin.

Obj. 3:  Further, if ignorance is a sin, this can only be in so far as it is voluntary.  But if ignorance is a sin, through being voluntary, it seems that the sin will consist in the act itself of the will, rather than in the ignorance.  Therefore the ignorance will not be a sin, but rather a result of sin.

Obj. 4:  Further, every sin is taken away by repentance, nor does any sin, except only original sin, pass as to guilt, yet remain in act.  Now ignorance is not removed by repentance, but remains in act, all its guilt being removed by repentance.  Therefore ignorance is not a sin, unless perchance it be original sin.

Obj. 5:  Further, if ignorance be a sin, then a man will be sinning, as long as he remains in ignorance.  But ignorance is continual in the one who is ignorant.  Therefore a person in ignorance would be continually sinning, which is clearly false, else ignorance would be a most grievous sin.  Therefore ignorance is not a sin.

On the contrary, Nothing but sin deserves punishment.  But ignorance deserves punishment, according to 1 Cor. 14:38:  “If any man know not, he shall not be known.”  Therefore ignorance is a sin.

I answer that, Ignorance differs from nescience, in that nescience denotes mere absence of knowledge; wherefore whoever lacks knowledge about anything, can be said to be nescient about it:  in which sense Dionysius puts nescience in the angels (Coel.  Hier. vii).  On the other hand, ignorance denotes privation of knowledge, i.e. lack of knowledge of those things that one has a natural aptitude to know.  Some of these we are under an obligation