
Turing has a number of tremendous achievements to his credit. His wartime codebreaking may be the most famous; but perhaps the most important was the idea of the Turing machine, the theoretical apparatus which defined computation and computers. It had two distinct consequences: on the one hand, it dealt with the Entscheidungsproblem, one of the key issues of 20^{th} century mathematics; on the other, it gave rise, via Turing’s famous (1950) paper, to the period of intense optimism about artificial intelligence which I referred to earlier as the ‘Turing era’ . The curious thing is that these two consequences of the Turing machine point in opposite, almost antithetical directions.  
How so? The Entscheidungsproblem, posed by Hilbert, asks whether there is any mechanical procedure for determining whether a mathematical problem is solvable. The universal Turing machine embodies and clarifies the idea of ‘mechanical’ calculation. It is a simple apparatus which prints or erases characters on a paper tape according to the rules it has been given. In spite of this extreme simplicity it can in principle carry out any mechanical computation. In theory, in fact, it can run an appropriate version of any computer program, including the ones being used to display this page. In many respects it appears to be an entirely realistic machine which could easily be put together, but it has certain other qualities which make it an impossible abstraction. For one thing, it has to have an infinite paper tape: for another, it has to be immune to malfunction, no matter how long it runs; and most fundamental of all, it has to operate with discrete states – it must switch from one physical configuration to another without any intervening halfway stages. These characteristics mean that it is really more like a complex function than a real machine. Nevertheless, all realworld computers owe their computerhood to their resemblance to it.  
The clear conception of computation which the Turing machine provided allowed Turing to show that the Entscheidungsproblem had to be answered in the negative – there is no general procedure which can deal with all mathematical problems, even in principle. In fact, Turing was slightly too late to claim full credit for this result, which had already been established by Alonzo Church using a different approach, The thing is, this result goes naturally with Gödel’s proof of the incompleteness of arithmetic in the sense that both establish limitations of formal algorithmic calculation. Both, therefore, suggest that the kind of computation performed by machines can never fully equal the thought processes of human beings (however those may work), which do not seem to suffer the same limitations. Gödel seems to have interpreted his own work this way. In fact there is some reason to think that Turing initially took a similar view. Andrew Hodges has pointed out that after completing his work on the Entscheidungsproblem, Turing attempted to produce a formal logic based on ordinals. It seems to have been the idea that this new, ordinalbased work would provide the basis for the kind of ‘intuitive’ reasoning which Turing machines couldn’t deliver – the kind human beings used to see the truth of Gödel statements. Only when these efforts failed, it seems, did Turing look for reasons to think that machinestyle computation might be good enough to deliver a real mind after all.  
Looked at again in this light, the 1950 paper seems more evasive and equivocal. It is a curious paper in many ways, with its playful tone and respectful mentions of ESP and Ada, Countess of Lovelace, but it also skirts the issue. Can machines think? Well, it says, let’s consider instead whether they can pass the Turing test . If they can, well, perhaps the original question is too meaningless to worry about.
But it surely isn’t meaningless: it’s partly because we believe that people really can think that our attitude to death is so different from our attitude to switching off the computer, for example.  
It seems possible, anyway, that Turing’s desire to believe that a mechanical mind was possible led him to seek ways around the negative implications of his own work. The logic of ordinals was one possibility: when that failed, the Turing Test was basically another, justifiying further work with Turingmachine style computers.
Had he lived, of course, he might eventually have changed his mind about his own Test, or found better ways of dealing with ‘intuition’. We’ll never know quite how much we lost when, punished for his homosexuality with oestrogen injections and expelled from further participation in Government work, he killed himself with a poisoned apple.
But it is a poignant thought that in the natural course of things he could still have been alive today.  
