  What does the mainstream of natural law theory intend by using the word “natural” in that name for the theory?  The shortest accurate answer is “of reason,” as in “the law of reason” or “the requirements of reason.” Aquinas is particularly clear and explicit that in this context, “natural” is predicated of something (say, a law, or a virtue) only when and because that of which it is predicated is in line with reason, practical reason, or practical reason’s requirements: see Finnis 1980, 35–6. Moreover, he employs, through all his works, a methodological axiom: X’s nature is understood by understanding X’s capacities, which are understood by understanding their act[uation]s, which are understood by understanding their objects. But the objects of chosen acts are the intelligible intrinsic goods (aspects of human flourishing) which we are directed to by practical reason’s first principles.  So the equation, in this context, of “natural” and “rational” and its cognates is no mere confusion, but grounded in a sophisticated distinction between ontology and epistemology: in the order of being, what is good and reasonable for us is a resultant of what is foundational, our given nature; but in the order of coming to know, our knowledge of our nature is in significant part a resultant of our understanding of what kinds of possible objects of choice are good.