Objection 1. It would seem that ignorance does not cause involuntariness. For "the involuntary act deserves pardon," as Damascene says (De Fide Orth. ii, 24). But sometimes that which is done through ignorance does not deserve pardon, according to 1 Corinthians 14:38: "If any man know not, he shall not be known." Therefore ignorance does not cause involuntariness.
Objection 2. Further, every sin implies ignorance; according to Proverbs 14:22: "They err, that work evil." If, therefore, ignorance causes involuntariness, it would follow that every sin is involuntary: which is opposed to the saying of Augustine, that "every sin is voluntary" (De Vera Relig. xiv).
Objection 3. Further, "involuntariness is not without sadness," as Damascene says (De Fide Orth. ii, 24). But some things are done out of ignorance, but without sadness: for instance, a man may kill a foe, whom he wishes to kill, thinking at the time that he is killing a stag. Therefore ignorance does not cause involuntariness.
On the contrary, Damascene (De Fide Orth. ii, 24) and the Philosopher (Ethic. iii, 1) say that "what is done through ignorance is involuntary."
I answer that, If ignorance causes involuntariness, it is in so far as it deprives one of knowledge, which is a necessary condition of voluntariness, as was declared above (Article 1). But it is not every ignorance that deprives one of this knowledge. Accordingly, we must take note that ignorance has a threefold relationship to the act of the will: in one way, "concomitantly"; in another, "consequently"; in a third way, "antecedently." "Concomitantly," when there is ignorance of what is done; but, so that even if it were known, it would be done. For then, ignorance does not induce one to wish this to be done, but it just happens that a thing is at the same time done, and not known: thus in the example given (Objection 3) a man did indeed wish to kill his foe, but killed him in ignorance, thinking to kill a stag. And ignorance of this kind, as the Philosopher states (Ethic. iii, 1), does not cause involuntariness, since it is not the cause of anything that is repugnant to the will: but it causes "non-voluntariness," since that which is unknown cannot be actually willed. Ignorance is "consequent" to the act of the will, in so far as ignorance itself is voluntary: and this happens in two ways, in accordance with the two aforesaid modes of voluntary (3). First, because the act of the will is brought to bear on the ignorance: as when a man wishes not to know, that he may have an excuse for sin, or that he may not be withheld from sin; according to Job 21:14: "We desire not the knowledge of Thy ways." And this is called "affected ignorance." Secondly, ignorance is said to be voluntary, when it regards that which one can and ought to know: for in this sense "not to act" and "not to will" are said to be voluntary, as stated above (Article 3). And ignorance of this kind happens, either when one does not actually consider what one can and ought to consider; this is called "ignorance of evil choice," and arises from some passion or habit: or when one does not take the trouble to acquire the knowledge which one ought to have; in which sense, ignorance of the general principles of law, which one to know, is voluntary, as being due to negligence. Accordingly, if in either of these ways, ignorance is voluntary, it cannot cause involuntariness simply. Nevertheless it causes involuntariness in a certain respect, inasmuch as it precedes the movement of the will towards the act, which movement would not be, if there were knowledge. Ignorance is "antecedent" to the act of the will, when it is not voluntary, and yet is the cause of man's willing what he would not will otherwise. Thus a man may be ignorant of some circumstance of his act, which he was not bound to know, the result being that he does that which he would not do, if he knew of that circumstance; for instance, a man, after taking proper precaution, may not know that someone is coming along the road, so that he shoots an arrow and slays a passer-by. Such ignorance causes involuntariness simply.
From this may be gathered the solution of the objections. For the first objection deals with ignorance of what a man is bound to know. The second, with ignorance of choice, which is voluntary to a certain extent, as stated above. The third, with that ignorance which is concomitant with the act of the will.
Source: Summa Theologica (New Advent)