Cargraves squinted up at it. "Might. But why waste film? They get better pictures of it from the earth. Wait until we go into a free orbit and swing ship. Then you can get some really unique pics—the earth from space. Or wait until we swing around the moon."
"That's what I really want! Pictures of the other side of the moon."
"That's what I thought." Cargraves paused a moment and then added, "But how do you know you can get any?"
"But—Oh, I see'. what you mean. It'll be dark on that side."
"That's not exactly what I meant, although that figures in, too, since the moon will be only about three days past ‘new moon' -- ‘new moon,' that is, for the other side. We'll try to time it to get all the pics you want on the trip back. But that isn't what I mean: how do you know there is any back side to the moon? You've never seen it. Neither has any one else, for that matter."
"But- there has to -I mean, you can see..."
"Did I hear you say there wasn't any other side to the moon, Doc?" It was Ross, whose head had suddenly appeared beside Cargraves'.
"Good morning, Ross. No, I did not say, there was no other side to the moon. I had asked Art to tell me what leads him to think there is one."
Ross smiled. "Don't let him pull your leg, Art. He's just trying to rib you."
Cargraves grinned wickedly. "Okay, Aristotle, you picked it. Suppose you try to prove to me that there is a far side to the moon."
"It stands to reason."
"What sort of reason? Have you ever been there? Ever seen it?"
"No, but-"
"Ever met anybody who's ever seen it? Ever read any accounts by anybody who claimed to have seen it?"
"No, I haven't, but I'm sure there is one."
"Why?"
"Because I can see the front of it."
"What does that prove? Isn't your experience, up to now, limited to things you've seen on earth? For that matter I can name a thing you've seen on earth that hasn't any back side."
"Huh? What sort of a thing? What are you guys talking about?" It was Morrie this time, climbing up on the other side.
Art said, "Hi, Morrie. Want your seat?"
"No, thanks. I'll just squat here for the time being." He settled himself, feet dangling. "What's the argument?"
"Doc," Ross answered, "is trying to prove there isn't any other side to the moon."
"No, no, no," Cargraves hastily denied. "And repeat ‘no.' I was trying to get you to prove your assertion that there was one. I was saying that there was a phenomenon even on earth which hasn't any back side, to nail down Ross's argument from experience with other matters—even allowing that earth experience necessarily applies to the moon, which I don't."
"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.