The Brain paused for a long minute before continuing. "I have studied many of the writings of the Earth, and am quite certain that I know more about its history than its average citizen. Do you recall the evidence found concerning the Cro-Magnon man of Earth's prehistoric ages? It seems that the so-called Neanderthal man was the animal that most nearly approached the present homo sapiens, until suddenly--as such things are reckoned--he was supplanted by another, much more advanced species of man, the Cro-Magnon. My research leads me to believe that those Cro-Magnon men migrated from our world to yours!" A dozen questions sprang to Ostby's mind concerning this fascinating theory, but he put them aside impatiently. He was a man with a bulldog tenacity of purpose, and he had no intention of wasting time on questions prompted by idle curiosity. "That's a very interesting theory," he said, "and I would like to discuss it more fully some other time. But for now, are you telling me that the 'door' is a natural phenomenon?" "Not the present 'door,'" the Brain replied. "It was created, approximately twenty years ago, by the concentration and intellectual power of one mind--my own!" "But how did you do it?" "I don't know how much knowledge you have of physics," the Brain said slowly. "The explanation is a bit technical for the untrained man to understand. However, I'll explain it as simply as I can. "Matter, as you probably know, is made up of tiny electrified bodies called electrons. When measurements were made it was found that the whole mass of the electron is due to its electrical charge. The inevitable conclusion is that the material universe is not the substantial, objective thing it was formerly thought to be. Matter is a completely spectral thing with no actual substance. The idea of substance must be replaced by that of behavior. "Thus, opening the 'door' became a problem of controlling that behavior in such a way as to create a refraction of the matter separating worlds. That is not as simple as it may sound because a mind, to be able to do it, must possess a thorough understanding of the forces it deals with. It must have a tremendous capacity for concentration, and its logic must be entirely uninfluenced by emotion. I believe it is safe to say that no other mind, before mine, has ever combined these qualities in sufficient degree to accomplish the deed." Strangely Ostby was not too surprised by this revelation. The makers of the capsule residing in the flesh of his left forearm had concluded, as a result of their studies, that the "door" might be the product of mind power. Their greatest mistake had been that it would take the combined power of at least eight brilliant minds to achieve the necessary matter refraction. Here, then, lay the end of his search, Ostby knew. He regretted that its conclusion must entail the death of the Brain.