284. In the previous Question we considered the internal principles of human acts--that is, habits, good and bad, from which they proceed. Now we shall turn to the external principles, good and bad, that move one to one’s acts. The external principle that moves to evil is the demon, who tempts us to sin; the external principle that moves to good is God, who instructs us by His law and helps us by His grace to fulfill it. Temptation has been discussed already, and grace belongs to Dogmatic Theology; the next Question to be considered, therefore, is Law. Art. 1: LAW IN GENERAL (_Summa Theologica_, I-II, qq. 90-92.) 285. Definition.--Law is an ordinance of the reason for the common good promulgated by him who has authority in the community. (a) It is an ordinance, that is, a command or prohibition which has obligatory and lasting force. Hence, advice is not a law, because not obligatory; a rule that binds only during the lifetime of the lawgiver or of those who received it is not strictly a law, because not enduring. (b) It is an ordinance of the reason, since the rule and standard of human acts is reason (see 64 sqq.). Hence, the arbitrary will of a ruler commanding what is against reason would not be law, but rather iniquity. (c) It is made for the common good, that is, it must tend to promote, directly or indirectly, general happiness, which is the end of society. Hence, the commands of a tyrant which benefit a few at the expense of public peace and prosperity are not truly laws. (d) It is made by him who has authority, that is, by the person or persons who have the lawmaking power according to the form of government. Hence, the decisions of an advisory body or the decrees of a usurper are not laws. (e) It is made by the proper authority in a community, that is, as here understood, in a self-sufficing community, which has its own means for attaining its end and is independent in its own order of other societies. Hence, the regulations made by parents for their family are not called laws, since the family is not a self-sufficing society. (f) It is an ordinance that has been promulgated, that is, brought to the notice of those whom it binds. Hence, a law that has been drawn up but not published as such, is not obligatory even for those who know of its existence. A law becomes obligatory, however, as soon as it has been promulgated, and the presumption then is that the law is known; but he who is inculpably ignorant is not guilty of formal sin if he breaks the law.