AISB Quarterly (Newsletter of the Society for the Study of Artificial Intelligence and Simulation of Behaviour), No. 74, Autumn 1990, pages 9-19. ISSN 0268-4179.
Revised version in P. J. R. Millican and A. Clark (editors), Machines and Thought: The Legacy of Alan Turing, Volume 1, Oxford University Press, 1996, pages 137-164. ISBN 0-19-823593-3.
This paper was originally presented at the Turing Colloquium held at the University of Sussex in April, 1990.