"The problem comes up because we ask the question in the wrong way. We supposed that solids were one thing and space quite another, or just nothing whatever. Then it appeared that space was no mere nothing, because solids couldn't do without it. But the mistake in the beginning was to think of solids and space as two different things, instead of as two aspects of the same thing. The point is that they are different but inseparable, like the front end and the rear end of a cat. Cut them apart, and the cat dies. Take away the crest of the wave, and there is no trough. Here is someone who has never seen a cat. He is looking through a narrow slit in a fence, and, on the other side, a cat walks by. He sees first the head, then the less distinctly shaped furry trunk, and then the tail. Extraordinary! The cat turns round and walks back, and again he sees the head, and a little later the tail. This sequence begins to look like something regular and reliable. Yet again, the cat turns round, and he witnesses the same regular sequence: first the head, and later the tail. Thereupon he reasons that the event head is the invariable and necessary cause of the event tail, which is the head's effect. This absurd and confusing gobbledygook comes from his failure to see that head and tail go together: they are all one cat. The cat wasn't born as a head which, sometime later, caused a tail; it was born all of a piece, a head-tailed cat. Our observer's trouble was that he was watching it through a narrow slit, and couldn't see the whole cat at once."