XLI., XLII., XLIII. But certain precautions are necessary to prevent us from being carried away by an opinion that civil customs, though founded upon just reasons, and received among many nations, are to be reckoned as a part of the law of nature. And in the next place, it is necessary to guard against enumerating as prohibitions of natural law, things which are not proved to be so, as certain kinds of marriages the taking of interest for the use of money, and other positive injunctions of the divine, or Mosaic law. The third rule is, to make an accurate distinction between general principles, such as the duty of living according to the dictates of reason, and those of a more particular though not less obvious meaning; as the duty of forbearing to take what belongs to another. To which many truths may be added though not quite so easy of apprehension: among which may be named the cruelty of that kind of punishment, which consists in revenge, delighting in the pain of another. This is a method of proof similar to that which occurs in mathematics, the process of which rises from self-evident truths to demonstrations, the latter of which, though not intelligible to all alike, upon due examination obtain assent. As then in matters of civil law, ignorance is deemed an excuse, so with respect to the law of nature, wherever infirmity of understanding forms an invincible obstruction to the knowledge of its rules, such infirmity may be alleged as a vindication. For as, in cases of unavoidable ignorance a great degree of the guilt of sin is removed; so it is in some measure softened wherever this ignorance subsists, though it may be owing to former negligence. And for this reason, Aristotle compares barbarians, in their rude, unformed state, to persons, whose appetites are rendered sickly by disease. Plutarch also observes that there are certain infirmities and disorders, which naturally infect the soul. Once for all, by way of conclusion we may add that wars undertaken to inflict punishment may be suspected of injustice, except there be manifest and enormous aggressions, with other conspiring causes, to vindicate nations for having recourse to arms. XLIV. The progress of the work has necessarily led to the consideration of offences against God; the propriety or impropriety of punishing which by force of arms is a fit subject of inquiry.