He paused for an instant, then added, "The fourth dimension is time, Rob. Not a new conception to scientists--think a moment--how would you define time?" "Time," I said, "Well, I read somewhere that time is what keeps everything from happening at once." * * * * * He did not smile. "Quite so. It is something in the universe of our consciousness along which we progress in measured rate from birth to death--from the beginning to the end. "We are living in a four-dimensional world--a world of length, breadth, thickness and time. The first three, to our human perception, have always been linked together. Time--I do not know why--seems to our minds something essentially different. Yet it is not. Our universe is a blending of all four. "Let me give you an example. That book there on the table--it exists because it has length, breadth and thickness. But Rob, it also has duration. It is matter, persisting both in space and in time. You see how the element time is involved? I'll go further. We know that two material bodies cannot occupy the same space at the same time. With three of the dimensions only--that is, if theoretically we remove the identical time-factor--they do not conflict. You're confused, Rob?" "I'm not quite sure what you're aiming at," I said. "You'll understand in a moment. Matter, as we know it, is merely a question of vibration. It is, isn't it?" "I know light is vibration," I responded. "And sound. And heat, and--" He interrupted me. "The very essence of matter is vibration. Do you know of what matter is composed? What is the fundamental substance? Let us see. First, we find matter is composed only of molecules. They are substances, vibrating in space. But of what are molecules composed? Atoms, vibrating in space. Atoms are substance. Of what are they composed?" "Electrons?" I said dubiously. "Protons and rings of electrons. Let us cling to substance, Rob. These electrons are merely negative, disembodied electricity--not matter, but mere vibration. They--these electrons--revolve around a central, positive nucleus. This then, is all the substance that matter has. But when you penetrate this inner nucleus, what do you find? Substance? Not at all. This proton, as they sometimes term it, this last inner strong-hold of substance, is itself a mere vortex--a whirlpool in space!" I groped at the thought. Matter, substance, everything tangible in my whole conscious universe, robbed of its entity, reduced to mere vibration in empty space. Vibration of what?