Whoever fights monsters should see to it that in the process he does not become a monster. And when you gaze long into an abyss, the abyss gazes also into you.
Wer mit Ungeheuern kämpft, mag zusehn, dass er nicht dabei zum Ungeheuer wird. Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein.

Philosophers
Friedrich Nietzsche
Prussian philosopher born in 1844 who became a Basel professor at 24, resigned due to illness, and produced "God is dead," the Ubermensch, and eternal recurrence -- ideas that shook modern philosophy to its core.
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Source: Jenseits von Gut und Böse (善悪の彼岸), Aphorismus 146Verified