The Natural Law is described as a law whose content is set by nature and is thus universally ingrained within each human person. As classically used, natural law refers to the use of reason to analyze human nature and deduce binding rules of moral behavior. The phrase natural law is opposed to the positive law (meaning "man-made law", not "good law"; cf. posit) of a given political community, society, or nation-state, and thus can function as a standard by which to criticize that law.
There belongs to the natural law, first, certain most general precepts, that are known to all; and secondly, certain secondary and more detailed precepts, which are, as it were, conclusions following closely from first principles. As to those general principles, the natural law, in the abstract, can nowise be blotted out from men's hearts. But it is blotted out in the case of a particular action, insofar as reason is hindered from applying the general principle to a particular point of practice, on account of concupiscence or some other passion. But as to the other, i.e., the secondary precepts, the natural law can be blotted out from the human heart, either by evil persuasions, just as in speculative matters errors occur in respect of necessary conclusions; or by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices were not esteemed sinful.