One of the things Ford Prefect had always found hardest to understand about humans was their habit of continually stating and repeating the very very obvious, as in It’s a nice day, or You’re very tall, or Oh dear you seem to have fallen down a thirty-foot well, are you all right? At first Ford had formed a theory to account for this strange behaviour. If human beings don’t keep exercising their lips, he thought, their mouths probably seize up. After a few months’ consideration and observation he abandoned this theory in favour of a new one. If they don’t keep on exercising their lips, he thought, their brains start working. After a while he abandoned this one as well, as being obstructively cynical, and decided he quite liked human beings after all, but he always remained desperately worried about the terrible number of things they didn’t know about. _______________________________________________________

Douglas Adams (1995, S. 45)

Insofern sich die Sätze der Mathematik auf die Wirklichkeit beziehen, sind sie nicht sicher, und insofern sie sicher sind, beziehen sie sich nicht auf die Wirklichkeit.
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Albert Einstein (1921/1991, S. 196ff)