There are some helpful insights to be found in the Catechism’s brief section on natural law (§§1954-1960).
“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” (1954).
“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” (1955).
“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” (Ibid.).
“For there is a true law: right reason. It is in conformity with nature, is diffused among all men, and is immutable and eternal; its orders summon to duty; its prohibitions turn away from offense …. To replace it with a contrary law is a sacrilege; failure to apply even one of its provisions is forbidden; no one can abrogate it entirely” (1956, a quotation from Cicero).
“Even when it is rejected in its very principles, it cannot be destroyed or removed from the heart of man. It always rises again in the life of individuals and societies” (1957).
“Theft is surely punished by your law, O Lord, and by the law that is written in the human heart, the law that iniquity itself does not efface” (ibid., quoting St Augustine’s Confessions: II, 4, 9).
“The precepts of natural law are not perceived by everyone clearly and immediately. In the present situation sinful man needs grace and revelation so moral and religious truths may be known ‘by everyone with facility, with firm certainty and with no admixture of error.’ The natural law provides revealed law and grace with a foundation prepared by God and in accordance with the work of the Spirit” (1960).
Aquinas, Etc. 
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