Summa Theologiae Moralis, vol. I: De Principiis

S. Th. I-II, q. 91.

230. Law is divided first and immediately by reason of origin. And indeed by reason of origin together with subject, time and extension : one is eternal, supreme, independent and altogether universal ; another is temporal and particular, derived and participated from it in time. The latter, by reason of foundation or necessity together with promulgation, is divided into natural or necessary, which arises necessarily from the very essences of things and is promulgated by reason, and positive or free, which, added to natural law, has been posited beyond it by the free will of the legislator and needs special promulgation. These are distinguished also by reason of matter : for what is prescribed or forbidden by natural law is by its nature and necessarily good or evil ; what is prescribed and forbidden by positive law alone is therefore good or evil only because prescribed or forbidden. — But positive law by reason of its immediate author is divine (which by reason of time and perfection is old or new) or human, which is ecclesiastical or canonical, or civil or political, and by reason of origin, promulgation and expression is written or observed by use, i.e. introduced by custom.

Between natural law and divine positive law, mediates a hypothetical natural or natural-divine law, which namely partakes of both (more, however, of the natural than of the positive), because on some hypothesis which depends on the free will of God, the obligation arises necessarily : thus, revelation being given, the obligation of believing arises necessarily ; — between natural and human positive law mediates the law of nations, which in some way partakes of the necessity of natural law, and yet is introduced everywhere by universal custom.