“Euclid's postulates came from the Pythagorean theorem, not the other way around.”

—  Richard Hamming

The Unreasonable Effectiveness of Mathematics (1980)
Context: The idea that theorems follow from the postulates does not correspond to simple observation. If the Pythagorean theorem were found to not follow from the postulates, we would again search for a way to alter the postulates until it was true. Euclid's postulates came from the Pythagorean theorem, not the other way around.

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Richard Hamming 90
American mathematician and information theorist 1915–1998

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