Many arguments prove the existence of such a natural law…
XIII. Second, by the consent of nations, among whom (even the most savage) some law of the primitive nations obtains, from which even without a teacher they have learned that God should be worshipped, parents honored, a virtuous life be led and from which as a fountain have flowed so many laws concerning equity and virtue enacted by heathen legislators drawn from nature itself….
XV. …it is even most absurd that the rational creature as rational should not be subject to him [God] in the genus of morals and not be governed by him suitably to his nature (i.e., by moral means) by the establishment of a law. Hence it follows either that man ought to have been created independent by God (which is absurd) or that he has a natural law impressed upon him, in accordance with which he may be ruled by him.
XVII. Sixth, the testimonies of the most illustrious heathen philosophers, who bravely opposed themselves to that impious opinion [that there is no natural law] (such as Plato, Aristotle and the Stoics). Cicero…demonstrates by various weighty arguments “that we are born to justice, and that right is not established by opinion, but by nature. … Socrates…used to complain that this error was the source of all human vices, since if this were true all justice and piety would be swept away from the world. “For if nature does not ratify law,” says he, “then all the virtues may lose their sway. For what becomes of generosity, patriotism, or friendship? Where will the desire of benefitting our neighbors, or the gratitude that acknowledges kindness, be able to exist at all? … “True law, says he, “is right reason comformable to nature, universal, unchangeable, eternal, whose commands urge us to duty, and whose prohibitions restrain us from evil.” … “It is not one thing at Rome, and another at Athens; one thing today, and another tomorrow; but in all times and nations this universal law must forever reign, eternal and imperishable. It is the sovereign master and emperor of all beings. God himself is its author, its promulgator, its enforcer. And he who does not obey it flies from himself, and does violence to the very nature of man.”