"Now wait a minute; that sounds like some of those fourth dimension stories I've read. I recall that when I was younger, I read a murder mystery--something about a morgue, I think. At any rate, the murder was committed inside a locked room; no one could possibly have gotten in or out. One of the characters suggested that the murderer traveled through the fourth dimension in order to get at the victim. He didn't go through the walls; he went around them." The Senator puffed a match flame into the bowl of his pipe, his eyes on the younger man. "Is that what you're driving at?" "Exactly," agreed Camberton. "The fourth dimension. Time. You must go back in time to an instant when that wall did not exist. An infant has no shame, no modesty, no shield against the world. You must travel back down your own four-dimensional tube of memory in order to get outside it, and to do that, you have to know your own mind completely, and you must be _sure_ you know it. "For only if you know your own mind can you communicate with another mind. Because, at the 'instant' of contact, you _become_ that person; you must enter his own memory at the beginning and go _up_ the hyper-tube. You will have all his memories, his hopes, his fears, his _sense of identity_. Unless you know--beyond any trace of doubt--who _you_ are, the result is insanity." * * * * * The Senator puffed his pipe for a moment, then shook his head. "It sounds like Oriental mysticism to me. If you can travel in time, you'd be able to change the past." "Not at all," Camberton said; "that's like saying that if you read a book, the author's words will change. "Time isn't like that. Look, suppose you had a long trough filled with supercooled water. At one end, you drop in a piece of ice. Immediately the water begins to freeze; the crystallization front moves toward the other end of the trough. Behind that front, there is ice--frozen, immovable, unchangeable. Ahead of it there is water--fluid, mobile, changeable. "The instant we call 'the present' is like that crystallization front. The past is unchangeable; the future is flexible. But they both exist." "I see--at least, I think I do. And you can do all this?" "Not yet," said Camberton; "not completely. My mind isn't as strong as Wendell's, nor as capable. I'm not the--shall we say--the superman he is; perhaps I never will be. But I'm learning--I'm learning. After all, it took Paul twenty years to do the trick under the most favorable circumstances imaginable."