"'By "empty" you mean,' he said, 'where things don't knock up against you?' I told him No. I mean just empty, void, nothing but blank aether. "You don't knock up against things here, and the air is as good as you want. It can't be the lack of ordinary emptiness you feel." "I agreed that the word needed explaining. 'I suppose it is mental restlessness,' I said. 'I like to feel that for a tremendous distance there is nothing round me. Why, I don't know. Some men are built the other way and have a terror of space.' "He said that that was better. 'It is a personal fancy, and depends on your KNOWING that there is nothing between you and the top of the Dent Blanche. And you know because your eyes tell you there is nothing. Even if you were blind, you might have a sort of sense about adjacent matter. Blind men often have it. But in any case, whether got from instinct or sight, the KNOWLEDGE is what matters.' "Hollond was embarking on a Socratic dialogue in which I could see little point. I told him so, and he laughed. "'I am not sure that I am very clear myself. But yes--there IS a point. Supposing you knew-not by sight or by instinct, but by sheer intellectual knowledge, as I know the truth of a mathematical proposition--that what we call empty space was full, crammed. Not with lumps of what we call matter like hills and houses, but with things as real--as real to the mind. Would you still feel crowded?' "'No,' I said, 'I don't think so. It is only what we call matter that signifies. It would be just as well not to feel crowded by the other thing, for there would be no escape from it. But what are you getting at? Do you mean atoms or electric currents or what?' "He said he wasn't thinking about that sort of thing, and began to talk of another subject. "Next night, when we were pigging it at the Geant cabane, he started again on the same tack. He asked me how I accounted for the fact that animals could find their way back over great tracts of unknown country. I said I supposed it was the homing instinct. "'Rubbish, man,' he said. 'That's only another name for the puzzle, not an explanation. There must be some reason for it. They must KNOW something that we cannot understand. Tie a cat in a bag and take it fifty miles by train and it will make its way home. That cat has some clue that we haven't.' "I was tired and sleepy, and told him that I did not care a rush about the psychology of cats. But he was not to be snubbed, and went on talking.