The Phantom Tollbooth
"But – " he started to say, and it got no further than that. For while he was about to say that he didn't think that that was quite fair (a thought to which the obstinate Soundkeeper might not have taken kindly) he suddenly discovered the way he would carry his little sound from the fortress. In the instant between saying the word and before it sailed off into the air he had clamped his lips shut – and the "but" was trapped in his mouth, all made but not spoken. (12.104)
"If you had high hopes, how would you know how high they were? And did you know that narrow escapes come in all different widths? Would you travel the whole wide world without ever knowing how wide it was? And how could you do anything at long last," he [the Dodecahedron] concluded, waving his arms over his head, "without knowing how long the last was? Why, numbers are the most beautiful and valuable things in the world. Just follow me and I'll show you." (14.52)
"Then each of you agrees that he will disagree with whatever each of you agrees with," said Milo triumphantly; "and if you both disagree with the same thing, then aren't you really in agreement?"
"I'VE BEEN TRICKED!" cried the Mathemagician helplessly, for no matter how he figured, it still came out just that way. (16.56-57)