The Old Catholic Encyclopedia is out of copyright due to its age, therefore it is now in public domain and Kevin Knight has put it online for everyone’s use. He also has other public domain items such as writings of early Church fathers (not sure whose editing and translating were used on these) and other church documents.
Here is a link to the section in the Catechism that speaks of the natural law - #1954-1960 vatican.va/archive/ENG0015/_P6U.HTM#141954 Man participates in the wisdom and goodness of the Creator who gives him mastery over his acts and the ability to govern himself with a view to the true and the good.
The natural law expresses the original moral sense which enables man to discern by reason the good and the evil, the truth and the lie:
[INDENT]The natural law is written and engraved in the soul of each and every man, because it is human reason ordaining him to do good and forbidding him to sin . . . But this command of human reason would not have the force of law if it were not the voice and interpreter of a higher reason to which our spirit and our freedom must be submitted
The actual expression “natural law” is not in Scripture, but it is what St. Paul is referring to in his letter to the Romans: Romans 2:14-15 When Gentiles who have not the law do by nature what the law requires, they are a law to themselves, even though they do not have the law. They show that what the law requires is written on their hearts, while their conscience also bears witness and their conflicting thoughts accuse or perhaps excuse them