"Whoops! Slow up! Take the last one first. Don't natural laws apply anywhere in the universe?"

"Pure assumption, unproved."

"But astronomers make predictions, eclipses and such, based on that assumption—and they work out."

"You've got it backwards. The Chinese were predicting eclipses long before the theory of the invariability of natural law was popular. Anyhow, at the best, we notice certain limited similarities between events in the sky and events on earth. Which has nothing to do with the question of a back side of the moon which we've never seen and may not be there."

"But we've seen a lot of it," Morrie pointed out.

"I get you," Cargraves agreed. "Between librations and such—the eccentricity of the moon's orbit and its tilt, we get to peek a little way around the edges from time to time and see about 6o per cent of its surface—if the surface is globular. But I'm talking about that missing 40 per cent that we've never seen."

"Oh," said Ross, "you mean the side we can't see might just be sliced off, like an apple with a piece out of it. Well, you may be right, but I'll bet you six chocolate malts, payable when we get back, that you're all wet."

"Nope," Cargrave answered, "this is a scientific discussion and betting is inappropriate. Besides, I might lose. But I did not mean anything of the slice-out-of-an-apple sort. I meant just what I said: no back side at all. The possibility that when we swing around the moon to look at the other side, we won't find anything at all, nothing, just empty space-that when we try to look at the moon from behind it, there won't be any moon to be seen—not from that position. I'm not asserting that that is what we will find; I'm asking you to prove that we will find anything."

"Wait a minute," Morrie put in, as Art glanced wildly at the moon as if to assure himself that it was still there—it was! "You mentioned something of that sort on earth—a thing with no back. What was it? I'm from Missouri."

"A rainbow. You can see it from just one side, the side that faces the sun. The other side does not exist."

"But you can't get behind it."

"Then try it with a garden spray some sunny day. Walk around it. When you get behind it, it ain't there."

"Yes, but Doc," Ross objected, "you're just quibbling. The cases aren't parallel. A rainbow is just light waves; the moon is something substantial."

"That's what I'm trying to get you to prove, and you haven't proved it yet. How do you know the moon is substantial? All you have ever seen of it is just light waves, as with the rainbow."

Ross thought about this. "Okay, I guess I see what you're getting at. But we do know that the moon is substantial; they bounced radar off it, as far back as ‘46."

"Just light waves again, Ross. Infra-red light, or ultra-shortwave radio, but the same spectrum. Come again."

"Yes, but they bounced."

"You are drawing an analogy from earth conditions again. I repeat, we know nothing of moon conditions except through the insubstantial waves of the electromagnetic spectrum."

"How about tides?"

"Tides exist, certainly. We have seen them, wet our feet in them. But that proves nothing about the moon. The theory that the moon causes the tides is a sheer convenience, pure theory. We change theories as often as we change our underwear. Next year it may be simpler to assume that the tides cause the moon. Got any other ideas?"

Ross took a deep breath. "You're trying to beat me down with words. All right, so I haven't seen the other side of the moon. So I've never felt the moon, or taken a bite out of it. By the way, you can hang on to the theory that the moon is made of green cheese with that line of argument."

"Not quite," said Cargraves. "There is some data on that, for what it's worth. An astronomer fellow made a spectrograph of green cheese and compared it with a spetcrograph of the moon. No resemblance."

Art chortled. "He didn't, really?"

"Fact. You can look it up."

Ross shrugged. "That's no better than the radar data," he said correctly. "But to get on with my proof. Granted that there is a front side to the moon, whatever it's nature, just as long as it isn't so insubstantial that it won't even reflect radar, then there has to be some sort of a back, flat, round, square, or wiggly. That's a matter of certain mathematical deduction."

Morrie snorted.

Cargraves limited himself to a slight smile. "Now, Ross. Think it over. What is the content of mathematics?"

"The content of mathe-" He collapsed suddenly. "Oh."

"I guess I finally get it. Mathematics doesn't have any content. If we found there wasn't any other side, then we would just have to invent a new mathematics."

"That's the idea. Fact of the matter is, we won't know that there is another side to the moon until we get there. I was just trying to show you," he went on, "just how insubstantial a ‘common sense' idea can be when you pin it down. Neither ‘common sense' nor ‘logic' can prove anything. Proof comes from experiment, or to put it another way, from experience, and from nothing else. Short lecture on the scientific method—you can count it as thirty minutes on today's study time. Anybody else want breakfast but me? Or has the low weight made you queasy?" He started to climb out of his chair.

Ross was very thoughtful while they made preparations for breakfast. This was to be a proper meal, prepared from their limited supply of non-canned foods. The Galileo had been fitted with a galley of sorts, principally a hot plate and a small refrigerator. Dishes and knives, forks, and spoons could be washed, sparingly, with the water which accumulated in the dump of the air-conditioner, and then sterilized on the hot plate. The ship had everything necessary to life, even a cramped but indispensable washroom. But every auxiliary article, such as dishes, was made of zinc-reserve mass for the hungry jet.

They sat, or rather squatted, down to a meal of real milk, cereal, boiled eggs, rolls, jam, and coffee. Cargraves sighed contentedly when it had been tucked away. "We won't get many like that," he commented, as he filled his pipe. "Space travel isn't all it's cracked up to be, not yet."

"Mind the pipe, Skipper!" Morrie warned.

Cargraves looked startled. "I forgot," he admitted guiltily. He stared longingly at the pipe. "Say, Ross," he inquired, "do you think the air-conditioner would clean it out fast enough?"

"Go ahead. Try it," Ross urged him. "One pipeful won't kill us. But say, Doc-"

"Yes?"

"Well, uh, look—don't you really believe there is another side to the moon?"

"Huh? Still on that, eh? Of course I do."

"But it's just my opinion. I believe it because all my assumptions, beliefs, prejudices, theories, superstitions, and so forth, tend that way. It's part of the pattern of fictions I live by, but that doesn't prove it's right. So if it turns out to be wrong I hope I am sufficiently emotionally braced not to blow my top."

"Which brings us right back to study time," he went on. "You've all got thirty minutes credit, which gives you an hour and a half to go. Better get busy."

Art looked dumfounded. "I thought you were kidding Uncle. You don't mean to run such a schedule on the moon, do you?"

"Unless circumstances prevent. Now is a good time to work up a little reserve, for that matter, while there is nothing to see and no work to do."

Art continued to look astonished, then his race cleared. "I m afraid we can't, Uncle. The books are all packed down so far that we can't get at them till we land."

"So? Well, we won't let that stop us. A school," he quoted, "is a log with a pupil on one end and a teacher on the other. We'll have lectures and quizzes—starting with a review quiz. Gather round, victims."

They did so, sitting cross-legged in a circle on the hold bulkhead. Cargraves produced a pencil and a reasonably clean piece of paper from his always bulging pockets. "You first, Art. Sketch and describe a cyclotron. Basic review—let's see how much you've forgotten."