Natural law is the promulgation of the eternal moral law in all creation, especially in created persons, both in the nature of each created thing, and in the ordered relationship between created things.
Saint Thomas Aquinas: "Augustine says (De Lib. Arb. i, 6) that 'knowledge of the eternal law is imprinted on us.' "
Summa Theologica, I-II, Q. 90, A. 4.
Saint Thomas Aquinas: "The natural law is a participation in us of the eternal law…."
Summa Theologica, I-II, Q. 91, A. 2.