Summa Theologiae Moralis, vol. I: De Principiis

On Ignorance.

373. Does ignorance excuse from fault? — Ignorance on the part of the subject does not make it so that the law does not reach him in the first act, but it can make it so that it does not oblige in the second act and the transgressor is excused from fault. For invincible ignorance (antecedent or concomitant), whether of right or of law, or of fact determining the application of law, excuses from fault because it removes the voluntary : for we will nothing unless foreknown ; on the contrary vincible ignorance because it does not remove the voluntary, this nevertheless diminishes fault, unless it be affected for liberty of sinning or for having an excuse from sin (n. 82).

374. Does ignorance of law excuse from penalty? — Invincible ignorance, whether of right or of fact, in the internal forum excuses from penalty because it excuses from fault ; on the contrary vincible does not excuse from a proportioned penalty, it excuses however from an improportioned one, whence if the fault is light, a grave penalty is not incurred (can. 2199-2203)¹. In the external forum however, « ignorance or error about the law or penalty or about one's own fact or about another's notorious fact is not generally presumed ; about another's not notorious fact it is presumed, until the contrary is proved » (can. 16, § 2) ; and « an external violation of law being posited, fraud (i.e. deliberate will of violating the law) is presumed in the external forum, until the contrary is proved » (can. 2200, § 2). Which often does not happen, and therefore (if sufficient proof is lacking) sometimes one is bound to undergo the penalty for a just cause, the common good requiring it, at least when the judgment of a judge is added ; but then it is not a penalty properly so called but rather a burden to be undergone for the common good.

375. Does ignorance of penalty excuse from penalty? — Ignorance (even invincible) of the penalty alone does not excuse from a proportioned penalty, because reason dictates to each one that evil according to its nature is to be punished, e.g. homicide by death, theft by prison. Whence can. 2202, § 2 : « Ignorance of the penalty alone does not remove the imputability of the offense, but diminishes it somewhat ». Likewise it does not excuse from invalidation, for can. 16, § 1 : « No ignorance of irritant or inhabilitating laws excuses from them, unless something else is expressly said ». — Indeed not even ignorance of law exempts from any vindictive penalty latae sententiae in the external forum (can. 2229, § 3, 1°) ; whence can. 988 : « Ignorance of irregularities whether from offense or from defect and of impediments, does not excuse from them ».

Ignorance of law or penalty excuses however a) if the penalty supposes communion and therefore communication of the knowledge, as for medicinal [penalties] and censures, unless the ignorance is gross or sluggish (can. 2229, § 3, 1°) ; b) if the legislator requires knowledge or temerity, by which he does not wish to punish except him who has acted knowingly, rashly, deliberately, studiously, with presumption, or has dared (can. 2229, § 2), unless the ignorance is affected, because this in law is equated with knowledge and excuses from no penalty latae sententiae (can. 2229, § 1).