“From experience. That something is irrational is no argument against its existence, but rather a condition for it.”
Section IX, "Man Alone with Himself" / aphorism 515
Human, All Too Human (1878), Helen Zimmern translation
Original
Aus der Erfahrung. - Die Unvernunft einer Sache ist kein Grund gegen ihr Dasein, vielmehr eine Bedingung desselben.
Human, All Too Human (1878), Helen Zimmern translation
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Friedrich Nietzsche 655
German philosopher, poet, composer, cultural critic, and cl… 1844–1900Related quotes

“In principio it is impossible to prove from experiments that something is non-existent.”
The Magnetic Current http://adsabs.harvard.edu/abs/1941Sci....94..232E, Science, Volume 94, Issue 2436, pp. 232-233 (September, 1941)

The Theory Of Intuition In Husserls Phenomenology 1963, 1995 p. 9

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“That maxim, it’s not an argument against atheism—it’s an argument against foxholes.”
Source: Towing Jehovah (1994), Chapter 8, “Famine” (p. 213)

“The Pythagoreans discovered the existence of incommensurable lines, or of irrationals.”
This was, doubtless, first discovered with reference to the diagonal of a square which is incommensurable with the side, being in the ratio to it of √2 to 1. The Pythagorean proof of this particular case survives in Aristotle and in a proposition interpolated in Euclid's Book X.; it is by a reductio ad absurdum proving that, if the diagonal is commensurable with the side, the same number must be both odd and even. This discovery of the incommensurable... showed that the theory of proportion invented by Pythagoras was not of universal application and therefore that propositions proved by means of it were not really established. ...The fatal flaw thus revealed in the body of geometry was not removed till Eudoxus discovered the great theory of proportion (expounded in Euclid's Book V.), which is applicable to incommensurable as well as to commensurable magnitudes.
Achimedes (1920)