 Similar ideas were current in discussions of physicalism in the 1970s. As a counterexample to the psychophysical identity theory there was an ‘imitation man’, whose ‘brain-states exactly paralleled ours in their physico-chemical properties’ but who felt no pains and saw no colors (Campbell 1970). It was claimed that zombies are a counterexample to physicalism in general, and arguments were devised to back up the intuition that they are possible (Kirk 1974a, 1974b: he argued that physicalism commits one to the ‘Entailment Thesis’, by which the purely physical truths about the world entail mental truths by logical or conceptual necessity). Other kinds of systems were envisaged which behaved like normal human beings, or were even functionally like human beings, but lacked the ‘qualia’ we have (Block 1980a, 1980b, 1981; Shoemaker 1975, 1981). (See the entry on  qualia.  Roughly, qualia are the properties by which we classify experiences according to ‘what they are like’: what it is like to smell roasting coffee beans, for example. Even physicalists can consistently use this expression, although unlike dualists they take qualia to be physical.) The most systematic use of the zombie idea against physicalism is by David Chalmers (1996), whose contributions to the debate will be considered shortly.