The Science of Discworld II

lumps of stone can manage that.’
    The circle of wizards was sunk in gloom.
    â€˜And Dee’s people were doin’ quite well,’ muttered Ridcully. ‘Terrible beer, of course.’
    â€˜I suppose …’ Rincewind began.
    â€˜Yes?’ said the Archchancellor.
    â€˜Well … how about if we went back and stopped us from stopping the elves? And least we’d be back among people more interesting than cows.’
    â€˜Could we do that?’ said Ridcully to Ponder.
    â€˜I suppose so,’ said Ponder. ‘Technically, if we stop ourselves, then nothing will change, I assume. All this won’t have happened … I think. That is to say, it will have happened, because we’ll remember it, but then it won’t have happened.’
    â€˜Fair enough,’ said Ridcully. Wizards do not have a lot of patiencewith temporal paradoxes.
    â€˜ Can we stop ourselves?’ said the Dean. ‘I mean, how do we do it?’
    â€˜We’ll just explain the situation to us,’ said Ridcully. ‘We’re reasonable men.’
    â€˜Hah!’ said Ponder, and then looked up. ‘Oh, sorry, Archchancellor. I must have been thinking about something else. Do go on.’
    â€˜Ahem. If I was just about to fight elves, and someone who looked very much like me came up and told me not to, I’d assume it was an elvish trick,’ said the Lecturer in Recent Runes. ‘They can make you think they look like someone else, you know.’
    â€˜I’d know me if I saw me!’ said the Dean.
    â€˜Look, it’s easy,’ said Rincewind. ‘Trust me. Just tell yourself something about yourself that no one else could possibly know.’
    A worried look crossed the Dean’s face.
    â€˜Would that be wise?’ he said. Like many people, wizards often have secrets they don’t want themselves to know.
    Ridcully stood up. ‘We know it’ll work,’ he said, ‘because it’s already happened to us. Think about it. We must succeed in the end, because we know a species like this gets off the planet.’
    â€˜Yes,’ said Ponder, slowly, ‘and, then again, no.’
    â€˜What the hell does that mean?’ Ridcully demanded.
    â€˜Well … we’ve been to a future where it happens, certainly,’ said Ponder, twiddling his pencil nervously. ‘But there are other futures. The multiplex nature of the universe that allows it to absorb and cushion the effects of apparent paradoxes also means that nothing is certain, even if you know it is.’ He tried to avoid Ridcully’s stare. ‘We went to a future. At the moment, it exists only in our memories. Then, it was real. Now, it may never be. Look, Rincewind was telling me about some play writer he’s found, born around about Dee’s time but not in this branch of the universe. Yet we know he has an existence, because L-space contains all possible books in all possible histories. Do you see what I mean? Nothing is certain.’
    After a while, the Chair of Indefinite Studies said, ‘You know, I think I prefer the kind of universal law that says the third son of a king always gets the princess. They make sense.’
    â€˜The universe is so big, sir, that it obeys all possible laws,’ saidPonder. ‘For a given value of “teapot”.’
    â€˜Look, if we go back in time and talk to ourselves, why don’t we remember it?’ said the Lecturer in Recent Runes.
    Ponder sighed. ‘Because although it has already happened to us, it hasn’t yet happened to us .’
    â€˜I, er, tried something like that,’ said Rincewind. ‘While you were having your mussel soup just now I got Hex to send me back in time to warn myself to hold my breath when we landed in the river. It worked.’
    â€˜Did you hold your breath?’
    â€˜Yes, because I’ve warned myself.’
    â€˜So … was there any time anywhere where you didn’t hold your breath, thus giving yourself a mouthful of river water and causing you to go back to make sure you did ?’
    â€˜Probably there was, I think, but there isn’t now.’
    â€˜Oh, I see ,’ said the Lecturer in Recent Runes. ‘You know, it’s a good job we’re wizards, otherwise this time travel business could really be confusing …’
    â€˜At least we know that Hex can still make contact with us,’ said Ponder. ‘I’ll ask him