"Now this Law of Rule about Right and Wrong used to be called the Law of Nature."

"According to natural law moral theory, the moral standards that govern human behavior are, in some sense, objectively derived from the nature of human beings and the nature of the world" (Internet Encyclopedia of Philosophy). Lewis builds his case on the idea that there is a moral Law of Nature that all men everywhere basically believe in ... but no one can live up to it.

Public DomainCicero

When he refers to "the older thinkers," he may be thinking of Plato, Aristotle, and Cicero.