(a) when the law is negative (i.e., prohibitory), no inconvenience excuses from sin; for that which is forbidden by the Natural Law is always morally evil, and hence more to be shunned than even the greatest physical evil, or death. Example: One is obliged, under grave or light sin, as the case may be, to forfeit all temporal goods rather than blaspheme, murder, lie, etc.;
(b) when the law is affirmative (or mandatory), an inconvenience which, all things considered, is really and relatively grave, excuses from sin; for that which is commanded by the Natural Law is not always morally obligatory, but only at the right time and in the right circumstances (see 371), and hence its omission is not always morally evil. Examples: Sempronius vowed that he would go on foot to a place of pilgrimage, but when the day came he had a sprained ankle that would be badly injured if he walked. Caius received a jewel stolen from Balbus and promised that he would return it at once to the owner, but he finds that he cannot do so now without danger, either of the arrest of himself or of the one who took the jewel. Titus sees a person who has been seriously injured lying by the roadside, but he is tired, and neither gives help himself nor summons aid. In the first two cases the inconvenience is grave, and hence Sempronius may ride to the place of pilgrimage, and Caius may return the jewel to Balbus later; but the inconvenience of Titus is slight, and does not excuse him from sin.
319. Just as the Natural Law is unchangeable, because based on the unchangeable Eternal Law instituting the nature of man, so is it easily knowable, because it is promulgated by the light of reason. Hence: (a) invincible ignorance of the entire Natural Law is impossible in any person who has the use of reason; (b) complete forgetfulness of the Natural Law by mankind is impossible.
320. Those who have not the use of reason, either habitually (as children and the insane) or actually (as the intoxicated), may be invincibly ignorant of the Natural Law—for example, they may be unable to perceive even the difference between right and wrong. As to those who have the use of reason, they can be ignorant of the Natural Law only as follows:
(a) they cannot ever be invincibly ignorant of the most general precepts (such as “good is to be done,” “evil is to be avoided”), for since they know the difference between right and wrong, they must also perceive that which is contained in the concepts of right and wrong, viz., that the former is something desirable and which ought to be done, the latter something undesirable which must not be done;
(b) one cannot, as a rule, be invincibly ignorant of those precepts that are immediately inferred as necessary conclusions from the most general precepts (such as “that which was borrowed must be returned”), for the conclusion follows so easily from the manifest principle that only in exceptional cases could one be excused for not knowing its truth;
(c) one can, even as a rule, be ignorant of precepts that are inferred as necessary but very remote conclusions from the most general precepts, (such as “that which was borrowed must be returned at such a time or place, or in such a manner or condition”), for this conclusion is so far removed from its premise, and there are so many factors to be considered, that considerable knowledge and skill in reasoning are required for a correct judgment—things in which many people are lacking.
321. The Commandments of the Decalogue follow directly from the most general precepts of the Natural Law, and so to them may be applied what was said in the previous paragraph. Hence: (a) generally speaking, no person who has the use of reason can be invincibly ignorant of the Commandments. St. Paul blames the pagans as inexcusable in various sins committed against the Decalogue; (b) in special cases, a person who has the use of reason can be invincibly ignorant of one or more Commandments; for while the Commandments may be easily inferred by most persons from the common principles of right and wrong, there are sometimes involuntary impediments that hinder the right employment of reason. Thus, children and older persons whose mentality is undeveloped, although they know the difference between right and wrong, are frequently unable to draw the conclusion that follows from it (e.g., that one should not tell lies).
322. The Commandments regarding which invincible ignorance may most easily exist are: (a) those that deal with merely internal acts, for the malice of violating them is less apparent. Hence, many theologians admit that even among Christians the wickedness of sinful thoughts and desires may be inculpably unknown, at least when the wickedness of the corresponding external acts is also not known; (b) those that deal with the control of sensuality, for the impulse to inordinate acts is at times most vehement. Unde theologi sunt qui affirmant malitiam peccatorum externorum contra sextum invincibiliter ignorari posse, non solum apud infideles, sed etiam apud Christianos, ita quod ab adolescentibus facile ad tempus ignorari possit malitia mollitiei.
323. If a Commandment be applied to some particular case in which there are many circumstances to be considered, or some reason that appears to change the subject-matter of the law, even adults who have the perfect use of reason may be invincibly ignorant; for in such instances we are considering, not an immediate, but a remote conclusion from the general principles of Natural Law.