Founded in our nature and revealed to us by our reason, the moral law is known to us in the measure that reason brings a knowledge of it home to our understanding. The question arises: How far can man be ignorant of the natural law, which, as St. Paul says, is written in the human heart (Romans 2:14)? The general teaching of theologians is that the supreme and primary principles are necessarily known to every one having the actual use of reason. These principles are really reducible to the primary principle which is expressed by St. Thomas in the form: “Do good and avoid evil”. Wherever we find man we find him with a moral code, which is founded on the first principle that good is to be done and evil avoided. When we pass from the universal to more particular conclusions, the case is different. Some follow immediately from the primary, and are so self-evident that they are reached without any complex course of reasoning. Such are, for example: “Do not commit adultery”; “Honour your parents”. No person whose reason and moral nature is ever so little developed can remain in ignorance of such precepts except through his own fault.