 What exactly does this argument show? It is intended to tell against non-causal theories of action, which deny that reasons explain actions by causing them. There have been sophisticated attempts, on the behalf of non-causal theories of action explanation, to respond to this challenge (von Wright 1971; Wilson 1985; Ginet 1995; for a good overview, see Stoutland 1976; and see related discussion in  6.3.)  However, assuming the argument is successful, while it does establish mental efficacy of a kind, it does not by itself establish the interaction principle. Establishing that reasons explain actions by causing them, and that therefore reasons causally interact with actions, does not establish that reasons causally interact with physical events. Dualists who reject the identity of mental and physical events will surely object.