something connected with his classes," Fred shrugged. He went on searching the waste basket, giving his mother no hint that he had already found what he was searching for.

From the position of the paper in the waste basket he felt reasonably sure it had been recently written. It was probably a voicing of thoughts gained from the disappearance of Horace and John, because up to that time his father had assumed his theory was just an intellectual curiosity.

His father couldn't have asked himself if the observable universe might not be the universe unless something had happened to raise a doubt, or suggest an alternative as a possibility.

Mrs. Grant's interest lessened. She wandered about the room, perhaps reliving memories. It gave Fred a chance to put the piece of paper in his pocket so that when he put everything back in the waste basket his mother would dismiss the whole search.

There was, of course, the file with the entire theory in it. He knew the theory by heart, however, and had no need of that file.

"I think I'll go out for a while, Mom," he said.

"All right, Fred," she said disinterestedly.

Outside he climbed behind the wheel of his hot rod and sat there, making no motion to start the motor. He was thinking.

Mark Smythe had said that he overheard two of his fellow class-men discussing the theory, one of them remarking that, "It would be funny if we were here just because we were descended from a long line of people who believed this was the only place."

Could that be the key?

Take gravitation, for instance. If it were something that some vital part of you had to believe, and that vital part didn't believe, would the entire person go flying off into space?

What about inanimate matter? Did it have to believe too? And what about other forms of life?

Or was everything except human beings just part of the props?

He shook his head. That didn't seem like quite the right track. He took another.

The human mind builds up a picture of the outside universe through its senses. Sometimes its ideas are wrong. Right or wrong, inside everyone's mind is a universe, derived from the outside universe.

What if the outside universe were derived from something? Derived from what? The real, logically necessary universe? That could be. At least it seemed to have some value as a starting point.

He tried to reason from that point. Frustration grew in him. He wished he were older, had his university education behind him. There were so many things he couldn't begin to deal with.

Maybe he could take the entire problem to some of his father's friends. He shook his head over this thought. From all that had gone on it was too likely that the minute one of them discovered something that would be of help he would disappear before he could tell it!

That raised another point. Why didn't he himself vanish? What was there different about him?

A lot. His father had instilled in him a lot of the things he himself could only aspire to. Unbelief was the major thing. Or perhaps it was the other major thing, remembrance.

His father's voice came into consciousness, saying something he had said so many times it was grooved deeply in memory, even to the inflections of voice. "All psychoses and mental troubles are caused by walled-off unpleasant memories. The child who trains himself to recall all unpleasant things and deliberately associate them with the feeling that they are valuable lessons, but harmless, will grow up in perfect balance."

He smiled. He could let flow through consciousness, dozens of incidents he had taken up with his father.

He was definitely different than others around him. So different he had systematically disguised it by a front of accepted behavior—systematically and consciously, under his father's guidance.

There was a chance those differences made him safe. There was a chance those differences would make it possible for him to find out what caused the others to vanish, without he himself vanishing.

The other train of thought inserted itself into consciousness again. Was belief the key to the disappearances?

Mark Smythe hadn't paid attention when the theory was being explained. The others had undoubtedly lapped it up. The peculiar thing about the theory was that it was so logical and so inevitable that the mind tended to accept it, believe it to be true in spite of the evidence of the senses.

Let us suppose, Fred mused, that deep within the mind there is some matrix of thought that ties the human to this universe. A matrix that could conceivably be altered, and when altered would automatically shift the person to another universe that the altered matrix fitted.

The subconscious usually took time to absorb and react. That was another thing his father had taught him to observe. Learn something, and it takes from days to months for it to become lodged in the subconscious and to rise into operation naturally from there.

John Henderson had taken six weeks to vanish after having learned the theory. It had taken Horace Smith three and a half days, but he had had the added factor of Dr. Henderson's disappearance to trigger reactions. The theoretical physics class had taken three days exactly, and its vanishing had been a sort of group action or chain reaction, with intensely emotional reaction after the first student had vanished before the eyes of the others.

His own father, originator of the theory, had probably fallen into the trap of starting to believe after Horace had vanished, so it became a greater probability that the disappearance was related to knowledge of the theory. Seeing the students vanish had probably set up an emotional state where complete belief was precipitated.

In the whole series the only improbable part was that so many students would react in the same short time. That was partly nullified by the fact that it was a special class, and only high I.Q. students with excellent records were accepted. They would tend to be somewhat identical in reaction times.

He straightened up and stared through the windshield at the dark street. So there it was, the probable mechanism of vanishment. A system was fed into the conscious mind. The conscious mind accepted it. In due time that system was transferred down into the matrix that held the person in this reality or universe. Once there, it made the whole person transfer to a system where the altered matrix fitted. It might not be the system pictured in his father's theory. It might be a compromise system.

Where and when probably had no meaning in relation to the two systems. That was why, when the shift came, the person vanished instantly without any strange manifestations of any kind.

Was it reversible? If so, then some of those who had vanished would reappear eventually.

A sudden, startling thought made Fred sit up straight, his eyes shining with excitement. So far he had been safe mainly because he habitually didn't attach belief to anything. His other facet of difference might be the means of his testing this without real danger of vanishing.

Could he dredge up from the deepest layers of unconscious thought, the threads leading directly to the matrix that held him in his surroundings and learn consciously what it was?

A thought. He reflected on it, then decided before he made any decisions he would explore the other avenue, the one the police had naturally thought of.

Was there some person or persons unknown in back of the disappearances? Some non-human, perhaps? It could fit into the same theory of disappearance. Another universe, beings in that universe. Beings who perhaps didn't want knowledge of their universe to become known on this side of the veil.

If so, why hadn't they snatched him too? Maybe they didn't know he knew about the theory. He'd never talked about it to anyone. But his father had drilled it into him as a supreme example of the reasons why belief in anything was