  The wit and drama of Turing's ‘imitation game’ has attracted more fame than his careful groundwork. Turing's argument was designed to bypass discussions of the nature of thought, mind, and consciousness, and to give a criterion in terms of external observation alone. His justification for this was that one only judges that other human beings are thinking by external observation, and he applied a principle of ‘fair play for machines’ to argue that the same should hold for machine intelligence. He dramatised this viewpoint by a thought-experiment (which nowadays can readily be tried out). A human being and a programmed computer compete to convince an impartial judge, using textual messages alone, as to which is the human being. If the computer wins, it must be credited with intelligence.