It had been going so well. They almost seemed up to speed. This may have been what caused Ponder to act like the man who, having so far fallen a hundred feet without any harm, believes that the last few inches to the ground will be a mere formality.

'To use the classic metaphor, the important thing is not to kill your own grandfather,' he said, and smacked into the bedrock.

'What the hell would I want to do that for?' said Ridcully. 'I quite liked the old boy.'

'No, of course, I mean accidentally,' said Ponder. 'But in any case—'

'Really? Well, as you know, I accidentally kill people every day,' said Ridcully. 'Anyway, I don't see him around—'

'It's just an illustration, sir. The problem is cause and effect, and the point is—'

The point, Mister Stibbons, is that you suddenly seem to think everyone comes over all fratricidal when they go back in time. Now, if I'd met my grandfather I'd buy him a drink and tell him not to assume that snakes won't bite if you shout at them in a loud voice, information which he might come to thank me for in later life.'

'Why?' said Ponder.

'Because he would have some later life,' said Ridcully.

'No, sir, no! That'd be worse than shooting him!'

'It would?'

'Yes, sir!'

'I think there may be one or two steps in your logic that I have failed to grasp, Mister Stibbons,' said the Archchancellor coldly. 'I suppose you're not intending to shoot your own grandfather, by any chance?'

'Of course not!' snapped Ponder. 'I don't even know what he looked like. He died before I was born.'

'Ah-hah!'

'I didn't mean—'

'Look, we're a lot further back in time than that,' said the Dean. 'Thousands of years, he says. No one's grandfather is alive.'

'That's a lucky escape for Mister Stibbons senior, then,' said Ridcully.

'No, sir,' said Ponder. 'Please! What I was trying to get across, sir, is that anything you do in the past changes the future. The tiniest little actions can have huge consequences. You might... tread on an ant now and it might entirely prevent someone from being born in the future!'

'Really?' said Ridcully.

'Yes, sir!'

Ridcully brightened up. That's not a bad wheeze. There's one or two people history could do without. Any idea how we can find the right ants?'

'No, sir!' Ponder struggled to find a crack in his Archchancellor's brain into which could be inserted the crowbar of understanding, and for a few vain seconds thought he had found one. 'Because... the ant you tread on might be your own, sir!'

'You mean... I might tread on an ant and this'd affect history and I wouldn't be born?'

'Yes! Yes! That's it! That's right, sir!'

'How?' Ridcully looked puzzled. 'I'm not descended from ants.'

'Because...' Ponder felt the sea of mutual incomprehension rising around him, but he refused to drown. 'Well... er... well, supposing it... bit a man's horse, and he fell off, and he was carrying a very important message, and because he didn't get there in time there was a terrible battle, and one of your ancestors got killed – no, sorry, I mean didn't get killed—'

'How did this ant get across the sea?' said Ridcully.

'Clung to a piece of driftwood,' said the Dean promptly. 'It's amazing what can get even on to remote islands by clinging to driftwood, insects, lizards, even small mammals.'

'And then got up the beach and all the way to this battle?' said Ridcully.

'Bird's leg,' said the Dean. 'Read it in a book. Even fish eggs get transported from pond to pond on a bird's leg.'

'Pretty determined ant, then, really,' said Ridcully, stroking his beard. 'Still, I must admit stranger things have happened.'

'Practically every day,' said the Senior Wrangler.

Ponder beamed. They had successfully negotiated an extended metaphor.

'Only one thing I don't understand, though,' Ridcully added. 'Who'll tread on the ant?

'What?'

'Well, it's obvious, isn't it?' said the Archchancellor. 'If I tread on this ant, then I won't exist. But if I don't exist, then I can't have done it, so I won't, so I will. See?' He prodded Ponder with a large, good-natured finger. 'You've got some brains, Mister Stibbons, but sometimes I wonder if you really try to apply logical thought to the subject in hand. Things that happen stay happened. It stands to reason. Oh, don't look so downcast,' he said, mistaking – possibly innocently – Ponder's expression of futile rage for shameful dismay. 'If you get stuck with any of this compl'cated stuff, my door's always open. I am your Archchancellor, after all.'

'Excuse me, can we tread on ants or not?' said the Senior Wrangler peevishly.

'If you like.' Ridcully swelled with generosity. 'Because, in fact, history already depends on your treading on any ants that you happen to step on. Any ants you tread on, you've already trodden on, so if you do it again it'll be for the first time, because you're doing it now because you did it then. Which is also now.'

'Really?'

'Yes indeed.'

'So we should have worn bigger boots?' said the Bursar.

'Try to keep up, Bursar.'

Ridcully stretched and yawned. 'Well, that seems to be it,' he said. 'Let's try to get back to sleep, shall we? It's been rather a long day.'

Someone was keeping up.

After the wizards got back to sleep, a faint light, like burning marsh gas, circled over them.

He was an omnipresent god, although only in a small area. And he was omnicognizant, but just enough to know that while he did indeed know everything it wasn't the whole Everything, just the part of it that applied to his island.

Damn! He'd told himself the cigarette tree would cause trouble. He should have stopped it the moment it started growing. He'd never meant it to get out of hand like this.

Of course, it had been a shame about the other... pointy creature, but it hadn't been his fault, had it? Everything had to eat. Some of the things that were turning up on the island were surprising even him. And some of them never stayed stable for five minutes together.

Even so, he allowed himself a little smirk of pride. Two hours between the one called the Dean dying for a smoke and the bush evolving, growing and fruiting its first nicotine-laden crop. That was evolution in action.

Trouble was, now they'd start poking around and asking questions.

The god, almost alone among gods, thought questions were a good thing. He was in fact committed to people questioning assumptions, throwing aside old superstitions, breaking the shackles of irrational prejudice and, in short, exercising the brains their god had given them, except of course they hadn't been given them by any god, lord knows, so what they really ought to do was exercise those brains developed over millennia in response to the external stimuli and the need to control those hands with their opposable thumbs, another damn good idea that he was very proud of. Or would have been, of course, if he existed.

However, there were limits. Freethinkers were fine people, but they shouldn't go around thinking just anything.

The light vanished and reappeared, still circling, in the sacred cave on the mountain. Technically, he knew, it wasn't in fact sacred, since you needed believers to make a place sacred and this god didn't actually want believers.

Usually, a god with no believers was as powerful as a feather in a hurricane, but for some reason he'd not been able to fathom he was able to function quite happily without them. It may have been because he believed so fervently in himself. Well, obviously not in himself, because belief in gods was irrational. But he did believe in what he did.