Argument from Explanation: Externalists claim that psychological explanation can legitimately taxonomize mental states through factors that outstrip internal neurophysiology (Peacocke 1993; Shea, 2018). Burge observes that non-psychological sciences often individuate explanatory kinds relationally, i.e., through relations to external factors. For example, whether an entity counts as a heart depends (roughly) upon whether its biological function in its normal environment is to pump blood. So physiology individuates organ kinds relationally. Why can’t psychology likewise individuate mental states relationally? For a notable exchange on these issues, see Burge (1986, 1989, 1995) and Fodor (1987, 1991).