"Here he left his physics and became the mathematician. Among his mathematical discoveries had been certain curves or figures or something whose behaviour involved a new dimension. I gathered that this wasn't the ordinary Fourth Dimension that people talk of, but that fourth-dimensional inwardness or involution was part of it. The explanation lay in the pile of manuscripts he left with me, but though I tried honestly I couldn't get the hang of it. My mathematics stopped with desperate finality just as he got into his subject. "His point was that the constituents of Space moved according to these new mathematical figures of his. They were always changing, but the principles of their change were as fixed as the law of gravitation. Therefore, if you once grasped these principles you knew the contents of the void. What do you make of that?" I said that it seemed to me a reasonable enough argument, but that it got one very little way forward. "A man," I said, "might know the contents of Space and the laws of their arrangement and yet be unable to see anything more than his fellows. It is a purely academic knowledge. His mind knows it as the result of many deductions, but his senses perceive nothing." Leithen laughed. "Just what I said to Hollond. He asked the opinion of my legal mind. I said I could not pronounce on his argument but that I could point out that he had established no trait d'union between the intellect which understood and the senses which perceived. It was like a blind man with immense knowledge but no eyes, and therefore no peg to hang his knowledge on and make it useful. He had not explained his savage or his cat. 'Hang it, man,' I said, 'before you can appreciate the existence of your Spacial forms you have to go through elaborate experiments and deductions. You can't be doing that every minute. Therefore you don't get any nearer to the USE of the sense you say that man once possessed, though you can explain it a bit.'" "What did he say?" I asked.