doesn't mean a thing."

In the privacy of his study Martin Grant allowed himself to become excited. Fred had unwittingly come upon the vital clue to the two disappearances.

"Let's be clear about this," he said to himself, drumming on his desk nervously with his fingers. "Undoubtedly there's a connection between the vanishing and my theory. Both Horace and John arrived at something I've missed. And since my theory is exhaustive it can't be there. It must be—yes—it must be that they went a step farther." He pondered this a moment and added grudgingly, "A step I have missed." Then even more grudgingly, "An obvious step."

Automatically he opened a drawer and brought out a sheet of paper and a pencil. He wrote:

The theory contains within itself the proof that the universe must, by logical necessity, be constructed according to said theory. But observation and experience say this is not true.

He frowned at what he had written. This was the conclusion to which he had led both men. It was the conclusion upon which he had rested. They, obviously, had not rested there. They had gone on.

Under what he had written he wrote "Either:" on the left hand margin. Two inches under it he wrote, "Or:" Then he frowned at them. Suddenly he began writing rapidly after the Either: "The universe is not constructed according to logical necessity."

He hesitated, studying what he had written. Then, pursing his lips, he slowly wrote after the Or: "The observable universe is not the universe."

He nodded to himself. That hit at the core of the matter. A was X. B was not X. Therefore B was not A. Even though A and B were both called universe.

The question was, then—did the universe-of-logical-necessity exist? If so, what relationship did it have to the observable universe which quite obviously did exist?

Was that the question, the answer to which, gained in a moment of insight, had caused two men to utterly vanish?

He sighed with real regret. There was no way of knowing. Possibly a mechanical brain of the most advanced type could come out with a comprehensive picture after solving thousands of successive equations. Knowledge of simple basics was a far cry from a fully expanded system.

He pushed the sheet of paper away with a show of irritation. He was missing something. He was on the wrong track. Neither John nor Horace had the mental equipment to make more than a simple step beyond what he had accomplished. That was certain. It was equally certain that he could and would make it.

A startled expression appeared on his face. "Oh good lord!" he groaned. "My book. I must do something about that the first thing tomorrow. I—" He opened the drawer of his desk and took out an oblong of paper, the check against advance royalties. "I'll return this and not let them publish it. First thing in the morning. And from now on I resolve not to think of my theory or what caused John and Horace to vanish."

Folding the check neatly, he stuck it in his billfold and then started to read a book that interested him. He became engrossed in it. Half an hour later he came to enough to realize he was on safe ground, sigh with relief, and sink back into the trains of thought of the book.

It was a nice feeling to know he was safe.

It was Friday. The sun was shining brightly and the monotony of the blue sky was relieved here and there by filmy white clouds that gave it a pleasing three-dimensionalness.

But to Martin Grant there was something unreal about things. He decided it must be the light. Things stood out with too sharp clarity.

When he reached his office at the university he made arrangements for a substitute to take his ten o'clock class. Then he called the publishing company and made an appointment for ten-fifteen.

The hour from nine to ten seemed interminably long. He found it almost impossible to concentrate on such an unimportant subject as the application of tensor analysis to electronic circuits.

Ten o'clock came. He hurried to the parking lot and got in his car. It was real and comforting. But once again everything outside the windshield seemed too sharply defined.

He timed himself on the way across town to the publishing house. He would have to allow himself the same time to return for his eleven o'clock class. It took twelve minutes, plus another two to find a parking place. Two minutes from the car to the eleventh floor. He was frowning at his watch as he entered the publisher's office.

"Well, well, Dr. Grant! Glad to see you. I suppose you're anxious to see your book ready for market. It's coming very well. Just came back from the typesetters and is going into its first printing right away."

"Huh?" Martin said, completing his mental arithmetic and jerking into an awareness of his surroundings. "Oh, hello Mr. Browne," he said. "I was just figuring my time. I have an eleven o'clock class. I can only stay twenty-seven minutes. That gives me a three minute margin of error for traffic delays."

"I see," the publisher said, a twinkle in his eye. "As I was just saying, your book—"

"Oh yes, my book," Martin interrupted. "Just a minute." He took out his billfold and extracted the check, handing it to Mr. Browne.

"What's this for?" Mr. Browne asked, unfolding it. "Oh, the advance royalty check. Is something wrong with it?"

"I'm returning it," Martin said. "I can't let you publish my book."

"Can't let me publish it!" Browne exclaimed. "Why not? Don't tell me it infringes on someone else's copyright!"

"No. Nothing like that. I've merely decided I don't want it published. I'm returning your check."

"Well now, look!" Browne said. "We're a business establishment. You signed a contract. We signed one too. It protects both of us against just this sort of thing, you know." He studied Martin thoughtfully. "Sit down and relax," he invited. "I'm human. Tell me why you don't want it published. Maybe I might agree with you. We have over a thousand dollars tied up already in typesetting, but—"

Martin took the seat and glanced nervously at his watch to make sure the twenty-seven minutes hadn't elapsed.

"I've just changed my mind," he said curtly. "There are certain things—I'm the head of a department, you know. I must watch my reputation. That's it, my reputation. On due reflection I believe the book might hurt my standing."

"In what way?" Browne asked. "To tell you the truth, your other book did so well I didn't bother reading this one."

"There's a—" Martin brought himself up short. So Browne hadn't read it. So much the better. At least he wouldn't vanish. "I'm afraid," he added with a self-conscious chuckle that he hoped was genuine enough to pass, "the subject matter is a little too crackpottish in spots. That's the whole thing. It would reflect on my reputation."

"Maybe we could do a little editing on it," Browne said. "Cut out the parts you think crackpottish and substitute something else in those pages. I'll get the galleys and we can look at them."

"No!" Martin said. "No, I'm afraid we would have to cut out at least half of the book. No. The best thing is to forget it, but I'll make good your typesetting loss. I can pay you two hundred dollars right away and fifty dollars a month."

Browne lit a cigarette slowly, his eyes on Martin. "You're serious, aren't you," he said. "I'll tell you what we'll do. We'll let the whole thing ride for the present. Maybe later—"

"No!" Martin said. "It must never be published! It's very vital that it never be published."

"Okay," Browne said. "We won't publish it. We have the contract, but—we won't publish it."

"Thanks, very much," Martin said. "I must hurry back."

The publisher stared thoughtfully at the closed door after Martin had gone. He glanced down at the check.

Lecture room 304 was very large, capable of holding four hundred students in its