Cicero (106–43 BC) in De Republica wrote, “There is in fact a true law – namely, right reason – which is in accordance with nature, applies to all men and is unchangeable and eternal” (11:33). St. Thomas Aquinas defined the natural law as “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.”75
