Here, today's cosmology offers descriptions of how (Big Bang, various recent alternatives) and when (about 13 billion years ago), but not why. String theory, a recent innovation at the frontiers of physics, makes an interesting attempt at `why?' However, it leaves an even bigger `why?' unanswered: why string theory? Science develops the consequences of physical rules (`laws'), but it doesn't explain why those rules apply, or how such a set-up came to exist.

These are deep mysteries. At the moment, and probably for ever, they are not accessible to the scientific method. Here religions come into their own, offering answers to riddles about which science chooses to remain mute.

If you want answers, they are available.

Rather a lot of different ones, in fact. Choose whichever one makes you feel most comfortable.

Feeling comfortable, however, is not a criterion recognised by science. It may make us feel warm and fuzzy, but the historical development of scientific understanding shows that, time and again, warm and fuzzy is just a polite way of saying `wrong'.

Belief systems rely on faith, not evidence. They provide answers - but they don't provide any rational process to assess those answers. So although there are questions beyond the capacity of science to answer, that's mostly because science sets itself high standards for evidence, and holds its tongue when there isn't any. The alleged superiority of belief systems compared to science, when it comes to these deep mysteries, stems not from a failure of science, but from the willingness.of belief systems to accept authority without question.

So the religious person can take comfort that his or her beliefs provide answers to deep questions of human existence that are beyond the powers of science, and the atheist can take comfort that there is absolutely no reason to expect those answers to be right. But also no way to prove them wrong, so why don't we just coexist peacefully, stay off each other's turf, and each get on with our own thing? Which is easy to say but harder to do, especially when some people refuse to stick to their own turf, and use political means, or violence, to promote their views, when rational debate long ago demolished them.

Some aspects of some belief systems are testable, of course - the Grand Canyon is not evidence for Noah's flood, unless God is having a quiet joke at our expense, which admittedly would be a very Discworld thing to do. And if He is, then all bets are off, because His revealed word in [insert your preferred Holy Book] may well be a joke too. Other aspects are not testable: the deeper issues stray into intellectual territory where, in the end, you have to settle for whatever explanation your type of mind finds convincing, or just stop asking that kind of question.

But remember: what's most interesting about your beliefs, to anyone who does not share them, is not whether you're right - it's that what you believe is a window into the workings of your mind. `Ah, so you think like that, do you?'

This is where the great mystery of human existence leads, and where all explanations are true - for a given value of `true'.

THE WRONG TROUSERS OF TIME

THE GLASS GLOBE OF ROUNDWORLD had been installed on a pedestal in front of Hex by the time most of the senior faculty were up and milling around. They were always at a bit of a loose end when Second Breakfast had finished and it wasn't yet time for Elevenses, and this looked like entertainment.

`One asks oneself whether it really is worth saving,' said the Chair of Indefinite Studies. `It's had huge ice ages before, hasn't it? If the humans are too stupid to leave in time, then there's bound to be another interesting species around in half a million years or so.'

`But extinction is so ... sort of ... final,' said the Lecturer in Recent Runes.

`Yes, and we created their world and helped them become intelligent,' said the Dean. `We can't just let them freeze to death. It'd be like going on holiday and not feeding the hamster.'

A watchmaker as part of the watch, thought Ponder, adjusting the university's biggest omniscope; not just making the world, but tweaking it all the time ...

Wizards did not believe in gods. They didn't deny their existence, of course. They just didn't believe. It was nothing personal; they weren't actually rude about it. Gods were a visible part of the narrativium that made things work, that gave the world its purpose. It was just that they were best avoided close up.

Roundworld had no gods that the wizards had been able to find. But one that was built in ... that was a new idea. A god inside every flower and stone ... not just a god who was everywhere, but a god who was everywhere.

The last chapter of Theology of Species had been very impressive ...

He stood back. Hex had been busy all morning. So had the Librarian. Right now he was carefully dusting books and feeding them into Hex's hopper. Hex had mastered the secret of osmotic reading, normally only ever attempted by students.

And the Librarian had located a copy of the right Origin of Species, the book Darwin ought to have written. It had a picture of Darwin as a frontispiece. With a pointy hat he would have passed for a wizard anywhere. If it came to that, he could have passed for the Archchancellor.

Ponder waited until the wizards had settled down and opened their popcorn.

`Gentlemen,' he said, `I do hope you've all read my analysis ...?'

The wizards stared at him.

`I worked very hard on it all morning,' said Ponder. `And it was delivered to all your offices ....'

There was more staring.

`It had a green cover. ...'Ponder prompted.

The staring was quite intense now. Ponder gave in.

`Perhaps I should remind you of the important points?' he said.

The faces lit up.

`Just jog our memories,' said the Dean, cheerfully.

`I discussed alternate timelines in phase space,' [1] said Ponder. That was a mistake, he could see. His fellow wizards weren't stupid, but you had to be careful to shape ideas to fit the holes in their heads.

[1] Phase space, in a given context, is the space of everything that might have happened, not just what did. See The Science of Discworld.

`Two different legs in the Trousers of Time,' said Ponder. `In the year 1859, by the counting system commonly in use in that part of Roundworld, a book changed the way a lot of people thought about the world. It just happened to be the wrong book-'

`Prove it,' said the Chair of Indefinite Studies.

`Pardon, sir?'

`Well, correct me if I'm wrong, but supposing Theology of Species was the right book?' said the Chair.

`It muted scientific - that is, technomantic - progress for almost a hundred years, sir,' said Ponder, wearily. `It slowed down humanity's understanding of its place in the universe.'

`You mean that it was built by wizards and left on a shelf in a corridor?' said the Chair.

`That's only true on the outside, sir,' said Ponder. `My point is, something happened to Mr Darwin at some time in his life that caused him to write the wrong book. And it was wrong. Yes, it would have been the right book here on Discworld, sir. We know there is a God of Evolution.'

`That's right. Skinny old chap, lives on an island,' said Ridcully. `Decent sort, in his way. Remember? He was redesigning the elephant when we were there. With wheels, very clever. Very keen on beetles, too, as I recall.'

`So why'd Darwin write this theology book instead?' the Chair of Indefinite Studies persisted.

`I don't know, sir, but as I wrote on page 4, I'm sure you recall, it was the wrong book at exactly the right time. Nevertheless, it made sense. There was something in it for everyone. All the technomancers had to do was leave a place in their science for the local god, and all the priests had to give up were a few beliefs that none of the sensible ones believed anyway-'