Now, natural law theory as understood in the Aristotelian-Thomistic (A-T) tradition presupposes this understanding of natural objects. Human beings, like every other natural substance, have a nature or substantial form, and what is good for them -- what constitutes their flourishing -- is determined by the ends or final causes that follow upon having that sort of nature or substantial form. But just as we can normally determine the efficient causes of things without making reference to God, so too can we normally determine the final causes of things without making reference to God. And thus, just as we can do physics, chemistry, and the like without making reference to God, so too can we do ethics without making reference to God, at least to a large extent. For we can know what is good for a thing if we can know its nature, and we can know its nature by empirical investigation guided by sound (A-T) metaphysics. At least to a large extent, then, we can know what the natural law says just from the study of human nature and apart from any sort of divine revelation. That’s why it’s the natural law. Goodness, or at least the possibility of it, is just natural to us (as Philippa Foot might say).
UPDATE: Frank Beckwith comments on Dougherty here.