The following is taken from Volume 5 of Radio Replies by Frs. Leslie Rumble and Charles Carty.
513. Last year  a visiting non-Catholic American professor of philosophy, Mortimer J. Adler, said government must be based on the natural law, not on positive laws only. What did he mean by that? [N.b.: Adler was received into the Church in 1999.]
By positive laws only he meant legislation made by men merely because they happen to have political power to make laws, as if there were no higher laws than those they choose to make. Granted such an idea, men in power could impose any laws they pleased upon others. Might would be right. On the other hand, by natural law Dr. Adler meant the Will of the Creator who has not only endowed man with intellectual as well as physical gifts, but obliges him in conscience to live in accordance with his true nature and with natural moral principles. This means that there are certain rights and duties not originated by men themselves, which men cannot abolish, and which all men are obliged to observe. Might is not right. It must be subordinated to right and used only to maintain and defend it.
514. Does this natural moral law apply to non-Christians?
It applies to all human beings. Dr. Adler himself is a Jew, not a Christian. Even the Roman philosopher Cicero, who died 43 B.C., and knew nothing of the Christian religion, wrote in his book “De Republica,” 3:22, “True law is right reason in agreement with nature. It is of universal application, unchanging, everlasting. We cannot be freed from it by Senate or people. This law is not one thing at Rome and another at Athens, but is eternal and immutable, valid for all nations and for all times. God is the Author of it, its promulgator, and its enforcing judge. Whoever is disobedient to it is abandoning his true self and denying his own nature.” Observation shows us that every creature in the universe has been given its own nature by the Creator, in accordance with which it is intended to act. Irrational animals obey the laws of their nature by instincts which the Creator has implanted in them. Whether or not, in God’s providence, this came about by means of an evolutionary process is of no importance here. As contrasted with lower animals, human beings are endowed with reason and free will. Men are moral beings who, even if they are not Christians, are obliged to conform their lives voluntarily to the natural law of God as manifested by their own intelligence and dictated by their conscience. When they do anything which is wrong of its very nature, it is because they either have warped ideas or are acting against their natural conscience through sheer bad will.