 In the nineteenth century scientists began to think that physics was capable of explaining all physical events that were explicable at all. It seemed that every physical effect has a physical cause: that the physical world is ‘closed under causation’. The developing science of neurophysiology was set to extend such explanations to human behavior. But if human behavior is explicable physically, how does consciousness fit into the story? One response — physicalism (or materialism) — is to insist that consciousness too involves only physical processes. However, the phenomena of consciousness are hard to account for in those terms, and some thinkers concluded that something nonphysical must be involved. Given the causal closure of the physical, they were forced to conclude that consciousness has no effects on the physical world. On this view human beings are ‘conscious automata’, as T. H. Huxley put it: all physical events, human behavior included, are explicable in terms of physical processes; and the phenomena of consciousness are causally inert by-products — epiphenomena (see James 1890, Chapter 5). It eventually became clear that this view entailed that there could be purely physical organisms exactly like us except for lacking consciousness. G. F. Stout (1931) argued that if epiphenomenalism (the more familiar name for the ‘conscious automaton’ theory) is true,