Under the first head, there are four points of inquiry: 

(1) Whether ignorance is a cause of sin?

(2) Whether ignorance is a sin?

(3) Whether it excuses from sin altogether?

(4) Whether it diminishes sin?
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FIRST ARTICLE [I-II, Q. 76, Art. 1]

Whether Ignorance Can Be a Cause of Sin?

Objection 1:  It would seem that ignorance cannot be a cause of sin:  because a non-being is not the cause of anything.  Now ignorance is a non-being, since it is a privation of knowledge.  Therefore ignorance is not a cause of sin.

Obj. 2:  Further, causes of sin should be reckoned in respect of sin being a turning to something, as was stated above (Q. 75, A. 1).  Now ignorance seems to savor of turning away from something.  Therefore it should not be reckoned a cause of sin.

Obj. 3:  Further, every sin is seated in the will.  Now the will does not turn to that which is not known, because its object is the good apprehended.  Therefore ignorance cannot be a cause of sin.

On the contrary, Augustine says (De Nat. et Grat. lxvii) “that some sin through ignorance.”

I answer that, According to the Philosopher (Phys. viii, 27) a moving cause is twofold, direct and indirect.  A direct cause is one that moves by its own power, as the generator is the moving cause of heavy and light things.  An indirect cause, is either one that removes an impediment, or the removal itself of an impediment:  and it is in this way that ignorance can be the cause of a sinful act; because it is a privation of knowledge perfecting the reason that forbids the act of sin, in so far as it directs human acts.

Now we must observe that the reason directs human acts in accordance with a twofold knowledge, universal and particular:  because in conferring about what is to be done, it employs a syllogism, the conclusion of which is an act of judgment, or of choice, or an operation.  Now actions are about singulars:  wherefore the conclusion of a practical syllogism is a singular proposition.  But a singular proposition does not follow from a universal proposition, except through the medium of a particular proposition:  thus a man is restrained from an act of parricide, by the knowledge that it is wrong to kill one’s father, and that this man is his father.  Hence ignorance about either of these two propositions, viz. of the universal principle which is a rule of reason, or of the particular circumstance, could cause an act of parricide.  Hence it is clear that not every kind of ignorance is the cause of a sin, but that alone which removes the knowledge which would prevent the sinful act.  Consequently if a man’s will be so disposed that he would not be restrained from the act of parricide, even though he recognized his father, his ignorance about his father is not the cause of his committing the sin, but is concomitant with the sin:  wherefore such a man sins, not “through ignorance” but “in ignorance,” as the Philosopher states (Ethic. iii, 1).