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Quem Deus Vult Perdere, Prius Dementat

"Whom the gods would destroy, they first make mad" (spoken by Prometheus in Longfellow's poem The Masque of Pandora). Prior Latin version: Quem deus vult perdere, dementat prius (Boswell’s Johnson, 1783).

Translated.—Whom the Lord wishes to ruin, he first deprives of reason; or, “When God will punish, he will first take away the understanding.”
Geo. Herbert.—Jacula Prudentum. 2

Misattributed to Euripides; see further here: http://en.wikiquote.org/wiki/Euripides