Free Science of Discworld III by Terry Pratchett Page A

true,’ said Archchancellor Ridcully, narrowing his eyes. ‘That was an important point which needed to be made.’
    ‘Thank you, Archchancellor.’
    ‘And now it needs to be apologised for.’
    ‘Sorry, Archchancellor.’
    ‘Good. So, Mr Stibbons, what do—’
    There was a rattle from Hex’s writing engine. The spidery arms wove across the paper and wrote:
    +++ The Chair of Indefinite Studies is correct +++
    The wizards clustered round.
    ‘Right about what?’ said Ponder.
    +++ Charles Darwin of Theology of Species was for much of his life a Rector in the Church of England, a sub-set of the British nation +++ the computer scrawled. +++ The chief function of the priests of that religion at the time was to further the arts of archaeology, local history, lepidoptery, botany, palaeontology, geology and the making of fireworks +++
    ‘Priests did that?’ said the Dean. ‘What about the praying and so on?’
    +++ Some of them did that too, yes, although it was considered to be showing off. The God of the English did not require much in the way of sacrifice, only that people acted decently and kept the noise down. Being a priest in that church was a natural job for a young man of good breeding and education but no very specific talent. In the rural areas they had much free time. My calculations suggest that Theology of Species was the book that he was destined to write. In all the histories of third-level phase space, there is only one in which he writes The Origin of Species +++
    ‘Why is that?’ said Ponder.
    +++ The explanation is complex +++
    ‘Well, out with it,’ said Ridcully. ‘We’re all sensible men here.’
    Another piece of paper slid off Hex’s tray. It read: +++ Yes. That is the problem. You understand that every possibility of choice gives birth to a new universe in which that choice is manifest? +++
    ‘This is the Trousers of Time again, isn’t if said Ridcully.
    +++ Yes. Except that every leg of the Trousers of Time branches into many other legs, and so do those legs and every following leg, until everywhere is full of legs, which often pass through one another or join up again +++
    ‘I think I’m losin’ track,’ said Ridcully.
    +++ Yes. Language is not good at this. Even mathematics gets lost.But a little story might work. I will tell you the story. It will be not completely inaccurate +++
    ‘Go ahead,’ said Ridcully.
    +++ Imagine an unimaginably large number +++
    ‘Right. No problem there,’ said Ridcully, after the wizards had consulted among themselves.
    +++ Very well +++ Hex wrote. +++ From the moment that the Roundworld universe was made, it began to split into almost identical copies of itself, billions of times a second. That unimaginably large number represents all possible Roundworld universes that there are +++
    ‘Do all these universes really exist?’ said the Dean.
    +++ Impossible to prove. Assume that they do. In all those universes there are hardly any in which a man called Charles Darwin exists, takes a momentous ocean voyage, and writes a hugely influential book about the evolution of life on the planet. Nevertheless, that number is still unimaginably large +++
    ‘But imagined by a smaller imagination?’ said Ridcully. ‘I mean, is it half as many as the other unimaginable number?’
    +++ No. It is unimaginably large. But compared to the first number, it is unimaginably small +++
    The wizards debated this in whispers.
    ‘Very well,’ said Ridcully, at last. ‘Keep goin’ and we’ll kind of join in when we can.’
    +++ Even so, it is not so unimaginable as the number of universes in which the book was The Origin of Species . That number is quite strange and can only be imagined at all in very unusual circumstances +++
    ‘It’s unimaginably larger?’ said Ridcully.
    +++ Just unimaginably unique. The number one . Gentlemen. All by itself. One is one and all alone. One. Yes. In third-level phase space there is only one history where he gets on the boat,