The hypothesis behind invisible writings was laughably complicated. All books are tenuously connected through L-space and, therefore, the content of any book ever written or yet to be written may, in the right circumstances, be deduced from a sufficiently close study of books already in existence. Future books exist in potentia, as it were, in the same way that a sufficiently detailed study of a handful of primal ooze will eventually hint at the future existence of prawn crackers.

But the primitive techniques used hitherto, based on ancient spells like Weezencake's Unreliable Algorithm, had meant that it took years to put together even the ghost of a page of an unwritten book.

It was Ponder's particular genius that he had found a way around this by considering the phrase, 'How do you know it's not possible until you've tried?' And experiments with Hex, the University's thinking engine, had found that, indeed, many things are not impossible until they have been tried.

Like a busy government which only passes expensive laws prohibiting some new and interesting thing when people have actually found a way of doing it, the universe relied a great deal on things not being tried at all.

When something is tried, Ponder found, it often does turn out to be impossible very quickly, but takes a little while for this to really be the case – in effect, for the overworked laws of causality to hurry to the scene and pretend it has been impossible all along. Using Hex to remake the attempt in minutely different ways at very high speed had resulted in a high success rate, and he was now assembling whole paragraphs in a matter of hours.

'It's like a conjurin' trick, then,' Ridcully had said. 'You're pullin' the tablecloth away before all the crockery has time to remember to fall over.'