CCC 1955 The “divine and natural” law¹ shows man the way to follow so as to practice the good and attain his end. The natural law states the first and essential precepts which govern the moral life. It hinges upon the desire for God and submission to him, who is the source and judge of all that is good, as well as upon the sense that the other is one’s equal. Its principal precepts are expressed in the Decalogue. This law is called “natural,” not in reference to the nature of irrational beings, but because reason which decrees it properly belongs to human nature:
Where then are these rules written, if not in the book of that light we call the truth? In it is written every just law; from it the law passes into the heart of the man who does justice, not that it migrates into it, but that it places its imprint on it, like a seal on a ring that passes onto wax, without leaving the ring.² The natural law is nothing other than the light of understanding placed in us by God; through it we know what we must do and what we must avoid. God has given this light or law at the creation.³

1 GS 89 § 1.
2 St. Augustine, De Trin. 14, 15, 21: PL 42,1052.
3 St. Thomas Aquinas, Dec. praec. I.