Part 6 of Turing: a natural philosopher (1997)
The impact of wartime work on Turing's philosophyIn Alan Turing: the Enigma, I wrote that Christopher Morcom had died a second death in 1936, meaning that the concept of spirit freed from Laplacian determinism, which had stimulated Turing in 1930, would never be heard of again. It seemed to me strikingly clear that Turing's emotionally charged fascination with the problem of mind was the key to the mystery of how he, youthful outsider, had made a definitive and fundamental contribution with the concept of computability. By modelling the action of the human mind as a physical machine, he had brought radical new ideas into the world of symbolic logic. After 1936, it seemed, it was the powerful concept of the machine that had seized his imagination; and Turing's post-war writing would support this view. But in fact, his interpretation of ordinal logics in 1938 did leave the door open for something non-mechanical in the mind, and it now seems to me that Turing's views did not shift all at once in 1936 to espouse the total power of the computable.
My guess is that there was a turning point in about 1941. After a bitter struggle to break U-boat Enigma, Turing could then taste triumph. Machines turned and people carried out mechanical methods unthinkingly, with amazing and unforeseen results. This is when there was first talk between Turing and the young I. J. (Jack) Good about chess-playing algorithms. As I described in Alan Turing: the Enigma, this vision of mechanical intelligence must have stimulated great excitement; I would now go further and suggest that it was at this period that he abandoned the idea that moments of intuition corresponded to uncomputable operations. Instead, he decided, the scope of the computable encompassed far more than could be captured by explicit instruction notes, and quite enough to include all that human brains did, however creative or original. Machines of sufficient complexity would have the capacity for evolving into behaviour that had never been explicitly programmed. And it was at this period that he also lost interest in logic as a tool for probing reality — although it must be said that he retained a keen interest in theoretical computability within mathematics, being one of the first into the field when it was yoked to algebra in the late 1940s.
Possibly it was at the same time, or within months, that he also saw the megahertz speed of electronic components, and their reliable performance in the speech scrambling system used for telephone conversations between Roosevelt and Churchill. I suspect that it was only a short step to see the possibility of building a practical universal Turing machine in electronics. Certainly, by the end of the war, he was captivated by the prospect of exploring the scope of the computable on a universal Turing machine; and indeed he called it 'building a brain' when talking of his plans to his electronic engineer assistant.
Turing went to the National Physical Laboratory and worked on his detailed design for a computer , submitting it for approval in March 1946. Turing's Automatic Computing Engine (ACE), as it was dubbed, was chronologically second to the June 1945 EDVAC report bearing von Neumann's name, but in addition to the originality of its hardware design, it was ideologically independent: for (1) it was conceived from the outset as a universal machine for which arithmetic would be just one application, and (2) Turing sketched a theory of programming, in which instructions could be manipulated as well as data.
It was an intensely exciting idea that engineering could be done once for all, so that new problems would only need new paperwork. Of course, Turing had Bletchley Park as a model of how non-numerical and versatile machines might be urgently needed. Turing dramatized the range of possible operations with farsighted examples, of which the last was as follows:
Given a position in chess the machine could be made to list all the 'winning combinations' to a depth of about three moves on either side. This is not unlike the previous problem, but raises the question 'Can the machine play chess?' It could fairly easily be made to play a rather bad game. It would be bad because chess requires intelligence. We stated at the beginning of this section [i.e. when describing how programming is done] that the machine should be treated as entirely without intelligence. There are indications however that it is possible to make the machine display intelligence at the risk of its making occasional serious mistakes. By following up this aspect the machine could probably be made to play very good chess.This is a crucial statement of his thought, which I take to show that by 1945 Turing had come to believe computable operations had sufficient scope to include intelligent behaviour, and had firmly rejected the direction he had followed in studying ordinal logics. The puzzling reference to 'occasional serious mistakes' makes sense in the light of his later stated argument (considered below) for holding uncomputability to be irrelevant to intelligence, and proves that he must have pondered this question during the war.
 A. M. Turing, Proposed Electronic Calculator, National Physical Laboratory report (1946). Published in B. E. Carpenter and R. W. Doran (eds.), A. M. Turing's ACE Report of 1946 and other papers, MIT Press and Tomash Publishers (1986); and again in the Collected Works. (Crown Copyright 1946. Reproduced by permission of the Controller of HMSO.) See also the Bibliography on this site.
© 1997, Andrew Hodges.