“But I don’t want to go among mad people,” Alice remarked.
​”Oh, you can’t help that,” said the Cat: “we’re all mad here. I’m mad. You’re mad.”
​”How do you know I’m mad?” said Alice.
​”You must be,” said the Cat, “or you wouldn’t have come here.”

Lewis Carroll (1832-1898) English writer and mathematician [pseud. of Rev. Charles Lutwidge Dodgson]

Alice’s Adventures in Wonderland, ch. 6 “Pig and Pepper” (1865)
    (Source)