Stemming from the Stoics and Thomas Aquinas and reaching down to Adam Smith and Thomas Paine,
natural law has been the basis for the development of modern liberalism.
From "Grotius and the Natural Law Tradition", Literature of Liberty: A Review of Contemporary Liberal Thought, vol. 1, no. 4 October-December 1978. Can be found reprinted here:
https://oll.libertyfund.org/page/gro...-law-tradition