In contrast, he wished to argue that such an imitation principle did apply to ‘thinking’ or ‘intelligence’. If a computer, on the basis of its written replies to questions, could not be distinguished from a human respondent, then ‘fair play’ would oblige one to say that it must be ‘thinking’. This being a philosophical paper, he produced an argument in favour of adopting the imitation principle as a criterion.

Perhaps Alan also confronted the figure of his father, whose strength had somehow cancelled itself out, and who had not shown the marathon-runner’s quality of his son. Perhaps too there was a hidden disappointment that his father had never even tried to penetrate his concerns in the way that his mother, however irritatingly, attempted. If Alan’s friends heard him disparage his mother, they usually heard nothing of his father.

To avoid philosophical discussions about what ‘mind’ or ‘thought’ or ‘free will’ were supposed to be, he favoured the idea of judging a machine’s mental capacity simply by comparing its performance with that of a human being. It was an operational definition of ‘thinking’, rather as Einstein had insisted on operational definitions of time and space in order to liberate his theory from a priori assumptions.

But there again, he began to be beset by the bureaucrats who wanted him to be in at a certain time and work till five o’clock and leave. His procedure – and that of many others of us, let me say, not only he, who were really fascinated by the work – would be maybe to come in at midday and work until midnight the next day. And then, the problem being essentially solved, go off and rest up and not come back for 24 hours perhaps … they were getting much more work out of Alan Turing that way.