Metaphysica, quae circa divina versatur, answers pretty well to what is now called natural theology. In the order of sciences, then, natural theology comes last in philosophy, and presupposes a knowledge of all the rest. Ethics therefore do not presuppose natural theology, but are presupposed by it. It follows that ethical conclusions should be first reached by ethical reasonings, not by theological. This point is further explained in my Ethics and Natural Law, pp. 123-125.

Of God and His Creatures: 1.4