378. The obligation of all laws is not the same in kind, or degree. (a) Moral laws oblige one to do what is commanded or to omit what is forbidden, as a duty owed in conscience; hence, he who violates a law of this kind is guilty of moral fault. Penal laws oblige one to follow what they prescribe, if one would be free from guilt before the law and not liable in conscience to the penalty prescribed; hence, he who violates a penal law is guilty of juridical fault, and, if he further illegally resists the penalty, he becomes guilty also of moral fault. (b) Moral laws are not all of the same obligatory force, some of them obliging under grave sin, others under venial sin.
379. The following human laws are recognized as moral laws: (a) ecclesiastical laws, with few exceptions; (b) civil laws that confirm the Eternal or Divine Law, or that pertain directly to the common welfare, such as the laws that determine the duties of public officials, the rights of inheritance, etc.
380. The following human laws are generally regarded as merely penal: (a) ecclesiastical laws which expressly state that their observance is not required under pain of sin (e.g., the statutes of many Religious Orders); (b) civil laws of minor importance, or which the legislator imposes as a purely civil duty (e.g., some traffic regulations).
381. Moral laws oblige under grave sin if the two following conditions are present: (a) if the thing prescribed by the law is of great importance, because of its nature or circumstances; (b) if the lawgiver intended to impose a grave obligation.
382. A matter of light moment cannot be made the object of a law that binds under grave sin, for this would impose an intolerable burden, and would thus be contrary to the common good. What is unimportant in itself, however, may become important on account of its purpose or other circumstance.
383. The intention of the legislator to impose a grave moral obligation is recognized either: (a) from his own declaration, as when a church law is commanded under threat of the divine judgment; or (b) from circumstances that indicate such an intention, such as the gravity of the subject-matter of the law or the kind of penalty it prescribes, the general opinion of authorities, or the common practice of the community.
384. By obliging to the observance of what they command and the avoidance of what they forbid, laws indirectly oblige to what is necessary for such obedience. (a) Hence, the law obliges one to make use of the ordinary means for its fulfillment. Examples: He who has not used ordinary diligence to know the law, sins against the law if he violates its prescriptions. He who eats meat on a day of abstinence, because he neglected to provide himself with other food, is guilty of sin. (b) The law obliges one to use sufficient diligence in removing impediments to its fulfillment or dangers of its violation. Examples: The law of hearing Mass on Sunday obliges one not to stay up so late on Saturday that fulfillment will be impossible. The law of fasting obliges one to avoid dangerous occasions of its violation.
385. Interpretation.—Though laws are carefully framed as to language, doubts about their meaning will often arise—in ordinary cases, because of lack of understanding or changes of conditions, and in extraordinary cases, because from the circumstances the law seems inapplicable. Hence the need of explaining the law, which is done in ordinary cases by interpretation, in extraordinary cases by _epieikeia_ (see 411 sqq.).
386. Interpretation is a genuine explanation of the law, that is, one that states the meaning of the words of the law according to the intention the lawgiver had in mind when he chose them. It is of various kinds.
(a) According to the author from whom it proceeds, interpretation is authentic, if it comes from the lawgiver himself or from another authorized by him; it is usual, if it comes from common usage (i.e., from the manner in which the law is customarily observed); it is doctrinal, if it is made by learned men according to the rules of correct exegesis.