318. Moral impossibility is also defined as the inability to observe the law without serious injury or loss to oneself or a third party. Serious injuries are such as deprive some one of great goods, such as the use of reason, life, knowledge, friendship, health, reputation, property. Serious losses are such as prevent one from obtaining notable goods, The following rules indicate when grave inconvenience excuses, and when it does not excuse, from the guilt arising from the non-observance of Natural Law: (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: