He was going to vanish.
There alone lay the solution. He should have realized it. Once he vanished as had the others, he would have experience with the mystery. Personal experience. He would have all the data he required, instead of just data from the world he was in. If he had the ability to solve the problem of reappearance he would then be able to return, and go back again and show the others how to return.
The key to vanishing was belief, that quality of thought which his father had systematically weeded from his mind since earliest infancy. It might take time to overcome that, but it should be possible.
Already he believed some things. Or did he? Was it merely a realization that those things had a probability that approached certainty?
His patterns of thinking were too ingrained. His mind was too well integrated. If he became irritated the irritation immediately brought up the memories of the factors that made him react that way. If he became happy he consciously knew the pattern, stretching back to early infancy. It was ingrained within him.
He began to realize with a sinking sensation that he didn't actually know what belief was. If, in some way, it was present anywhere in his makeup, he didn't know how to recognize it.
His mental pattern was one of unbelief. Not disbelief, the believing that something isn't true; but unbelief, the using of something in the pragmatic sense for its workability.
He let his thoughts wander in the past. He could remember vaguely a moment when he had felt unreasoning terror, a sense of being lost. He could remember his father saying many times, "Belief is the lazy assuming that something is true." It is or it isn't, and the fundamental postulate of inductive logic tells us that its truth or lack of it is forever beyond our reach. So why reach for it? Use a theory if it works for you. Discard it if it doesn't. Don't use it even to the point of absurdity while clinging to a belief that it's true.
It was that way with facts, too. Something that happened or seemed to happen, needed no tag of belief attached to it. If you saw it happen it didn't necessarily happen. There was such a thing as illusion. Accept it as though it had happened—until events pointed otherwise.