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It was all completely artificial. The robot customers were attended by a robot waiter, a robot wine waiter and a robot maetre d’. The furniture was artificial, the tablecloth artificial, and each particular piece of food was clearly capable of exhibiting all the mechanical characteristics of, say, a pollo sorpreso, without actually being one.

And all participated in a little dance together - a complex routine involving the manipulation of menus, bill pads, wallets, cheque books, credit cards, watches, pencils and paper napkins, which seemed to be hovering constantly on the edge of violence, but never actually getting anywhere.

Slartibartfast hurried in, and then appeared to pass the time of day quite idly with the maetre d’, whilst one of the customer robots, an autorory, slid slowly under the table, mentioning what he intended to do to some guy over some girl.

Slartibartfast took over the seat which had been thus vacated and passed a shrewd eye over the menu. The tempo of the routine round the table seemed somehow imperceptibly to quicken. Arguments broke out, people attempted to prove things on napkins. They waved fiercely at each other, and attempted to examine each other’s pieces of chicken. The waiter’s hand began to move on the bill pad more quickly than a human hand could manage, and then more quickly than a human eye could follow. The pace accelerated. Soon, an extraordinary and insistent politeness overwhelmed the group, and seconds later it seemed that a moment of consensus was suddenly achieved. A new vibration thrilled through the ship.

Slartibartfast emerged from the glass room.

– Bistromathics, - he said. - The most powerful computational force known to parascience. Come to the Room of Informational Illusions.

He swept past and carried them bewildered in his wake.

The Bistromatic Drive is a wonderful new method of crossing vast interstellar distances without all that dangerous mucking about with Improbability Factors.

Bistromathics itself is simply a revolutionary new way of understanding the behaviour of numbers. Just as Einstein observed that time was not an absolute but depended on the observer’s movement in space, and that space was not an absolute, but depended on the observer’s movement in time, so it is now realized that numbers are not absolute, but depend on the observer’s movement in restaurants.

The first non-absolute number is the number of people for whom the table is reserved. This will vary during the course of the first three telephone calls to the restaurant, and then bear no apparent relation to the number of people who actually turn up, or to the number of people who subsequently join them after the show/match/party/gig, or to the number of people who leave when they see who else has turned up.

The second non-absolute number is the given time of arrival, which is now known to be one of those most bizarre of mathematical concepts, a recipriversexcluson, a number whose existence can only be defined as being anything other than itself. In other words, the given time of arrival is the one moment of time at which it is impossible that any member of the party will arrive. Recipriversexclusons now play a vital part in many branches of maths, including statistics and accountancy and also form the basic equations used to engineer the Somebody Else’s Problem field.

The third and most mysterious piece of non-absoluteness of all lies in the relationship between the number of items on the bill, the cost of each item, the number of people at the table, and what they are each prepared to pay for. (The number of people who have actually brought any money is only a sub-phenomenon in this field.)

The baffling discrepancies which used to occur at this point remained uninvestigated for centuries simply because no one took them seriously. They were at the time put down to such things as politeness, rudeness, meanness, flashness, tiredness, emotionality, or the lateness of the hour, and completely forgotten about on the following morning. They were never tested under laboratory conditions, of course, because they never occurred in laboratories - not in reputable laboratories at least.

And so it was only with the advent of pocket computers that the startling truth became finally apparent, and it was this:

Numbers written on restaurant bills within the confines of restaurants do not follow the same mathematical laws as numbers written on any other pieces of paper in any other parts of the Universe.

This single fact took the scientific world by storm. It completely revolutionized it. So many mathematical conferences got held in such good restaurants that many of the finest minds of a generation died of obesity and heart failure and the science of maths was put back by years.

Slowly, however, the implications of the idea began to be understood. To begin with it had been too stark, too crazy, too much what the man in the street would have said:

– Oh yes, I could have told you that, - about. Then some phrases like “Interactive Subjectivity Frameworks” were invented, and everybody was able to relax and get on with it.

The small groups of monks who had taken up hanging around the major research institutes singing strange chants to the effect that the Universe was only a figment of its own imagination were eventually given a street theatre grant and went away.

– In space travel, you see, - said Slartibartfast, as he fiddled with some instruments in the Room of Informational Illusions, - in space travel…

He stopped and looked about him.

The Room of Informational Illusions was a welcome relief after the visual monstrosities of the central computational area. There was nothing in it. No information, no illusions, just themselves, white walls and a few small instruments which looked as if they were meant to plug into something which Slartibartfast couldn’t find.

– Yes? - urged Arthur. He had picked up Slartibartfast’s sense of urgency but didn’t know what to do with it.

– Yes what? - said the old man.

– You were saying?

Slartibartfast looked at him sharply.

– The numbers, - he said, - are awful. - He resumed his search.

Arthur nodded wisely to himself. After a while he realized that this wasn’t getting him anywhere and decided that he would say “what?” after all.

– In space travel, - repeated Slartibartfast, - all the numbers are awful.

Arthur nodded again and looked round to Ford for help, but Ford was practising being sullen and getting quite good at it.

– I was only, - said Slartibartfast with a sigh, - trying to save you the trouble of asking me why all the ship’s computations were being done on a waiter’s bill pad.

Arthur frowned.

– Why, - he said, - were all the ship’s computations being done on a wait… -

He stopped.

Slartibartfast said:

– Because in space travel all the numbers are awful.

He could tell that he wasn’t getting his point across.

– Listen, - he said. - On a waiter’s bill pad numbers dance. You must have encountered the phenomenon.

– Well…

– On a waiter’s bill pad, - said Slartibartfast, - reality and unreality collide on such a fundamental level that each becomes the other and anything is possible, within certain parameters.

– What parameters?

– It’s impossible to say, - said Slartibartfast. - That’s one of them. Strange but true. At least, I think it’s strange, - he added, - and I’m assured that it’s true.

At that moment he located the slot in the wall for which he had been searching, and clicked the instrument he was holding into it.

– Do not be alarmed, - he said, and then suddenly darted an alarmed look at himself, and lunged back, - it’s…