Naturalism in metaethics is the view that moral properties are natural properties. Starting in the 1980s, a group of philosophers in the United States took up the project of assimilating moral epistemology and moral metaphysics to the epistemology and metaphysics of the sciences. By the 1980s, Cornell Realists had taken up the ideas as they worked to assimilate moral epistemology and moral metaphysics to the epistemology and metaphysics of the sciences. This chapter shows that Cornell Realists have adopted Philippa Foot's early resistance to internalism. Working in the 1960s to 1970s, Ruth Marcus, Saul Kripke, Hilary Putnam, David Kaplan, Ruth Millikan, and Richard Boyd developed so-called direct reference theories, according to which terms contributed the items for which they stood directly to the contents of the sentences and thought they expressed. Self-described nonreductive naturalists might, however, be trying to get at a metaphysical contrast when they use the label nonreductive.