Summa summae S. Thomae, Tomus II

Ignorance on the part of the will is divided into ignorance antecedent, concomitant and consequent¹ : and of ignorance taken in this sense the question is here. Antecedent ignorance is that which precedes every act of the will, so that it is not at all willed or voluntary; it is nevertheless the cause of willing or doing what would not be done if knowledge were present: as in one who, with sufficient diligence used, kills a friend, thinking invincibly that he was killing a stag, whom he would not have killed had he known him to be a friend. It is also called invincible. Yet antecedent and invincible ignorance do not coincide, for antecedent, as here taken, adds over and above simply invincible ignorance that it be the cause why something is done which would not be done were it absent; which does not belong to every invincible ignorance, as is clear in concomitant ignorance, which, as I shall say presently, is invincible, and yet not antecedent. Whence every antecedent ignorance is invincible, but not conversely.

Concomitant ignorance is that which also precedes every act of the will, nor is it at all willed, and therefore invincible, like the antecedent, but it is not the cause of willing or doing what would not be done if knowledge were present. It is clear in one who killed an enemy, thinking invincibly that he was killing a stag, whom he would just the same have killed had he known him to be an enemy.

Consequent ignorance is that which follows an act of the will, so that it is willed either directly or indirectly, and it is the cause why something is done which would not be done were it absent. That which is directly willed is called affected, as when someone directly and positively wills to be ignorant of something, whether that he may sin more freely, or that he may have an excuse in sins, or on account of something else; of this is understood that Job, XXI, 14 : The wicked said to God: Depart from us, and we will not the knowledge of thy ways. And that Psal., XXXV, 4 : He would not understand that he might do well. Indirectly willed it is, when someone, although he does not will directly and positively to be ignorant, yet neglects to learn; whence this ignorance is called voluntary or willed indirectly and in its cause, namely negligence of learning. Which negligence, if it be grave, such that either none or only very slight diligence was used according to the quality of the matter, the ignorance is called crass and supine; but if the negligence be slight, such that notable diligence was used in learning, yet not wholly sufficient, the ignorance is called light or lightest, according to the quantity of diligence used in learning, respectively to the quality of the thing to be learned. These things noted,

I say 1° : Antecedent ignorance causes the involuntary simply²; because the work proceeding from it is done without cognition, and is, as to the circumstance ignored, against the inclination, whether actual, or habitual and interpretative, of the agent; for he would not will to do it if the ignorance were absent. It is clear in him who, sufficient diligence being used, thinking invincibly that he was killing a stag, killed a friend. For although this killing, as to the substance of the act, and as it is a killing as such, be voluntary, yet not inasmuch as it is the killing of a friend; under that reason indeed it repugns the inclination of the slayer, and is done without cognition.

I say 2° : Concomitant ignorance does not make the voluntary simply; because the work proceeding from it is done without cognition, or with an ignorance which is neither voluntary in itself nor in its cause, since it is invincible. Nor does it make the involuntary positively and contrarily; because this work is not against the inclination of the agent, for it would be done just the same if cognition were present. But it makes the involuntary negatively, or not voluntary; because it is done without the cognition required for the voluntary. It is clear in him who, thinking invincibly that he was killing a beast, killed an enemy, whom he would just the same have killed had he known him.

I say 3° : Consequent ignorance causes the voluntary simply, because it itself is simply voluntary, whether directly in itself, or indirectly in its cause, namely negligence of learning. Therefore the work proceeding from it is voluntary simply; for he who wills the cause wills also the effect. If you say that it is done without cognition, I answer that it is done indeed without formal cognition and in itself, yet not without virtual cognition and in its cause, namely in the ignorance which could and ought to have been driven away. He however is said to know virtually who could and ought to have known and would not. Yet it causes the involuntary secundum quid; because the work following from it is against some sort of inclination of the agent, for if knowledge were present, it would not be done, as is supposed. But if you suppose the ignorance to be so affected that, were it absent, the work would nonetheless be done; as if, e.g., someone rejects knowledge, not that with it present he may abstain from sin, but that he may sin without remorse: I confess that then it will not be involuntary secundum quid, but also neither will it be consequent ignorance in the sense of the conclusion, which is precisely that it be the cause why that is done which would not be done were it absent.

From what has been said you will gather that antecedent ignorance altogether excuses from sin, because it causes the involuntary simply: but concomitant does not altogether excuse, because it does not take away all willingness, whether habitual or actual conditioned to sinning; nor yet accuse, because it is involuntary and invincible. Whence, in the case posited, he who kills an enemy, thinking invincibly that he kills a beast, remains always guilty of internal homicide, on account of the evil will of killing the enemy, if he knew him; he is nevertheless excused from external homicide and from the punishments and censures, which are inflicted only on account of the external work, because the external work is not voluntary. Consequent however causes sin, because it causes the voluntary simply; it diminishes nevertheless, because it causes the involuntary secundum quid, and the lighter it is the more it diminishes, so that sometimes it makes venial out of mortal.

What I have said here of ignorance can be said of inadvertence, which is a kind of actual ignorance, or at least is equated to ignorance in ordine to the voluntary. Let these things suffice now on the present matter, for there will be further mention of it when treating of the causes of sins.