Whether we believe the Greek poet, “it is sometimes even pleasant to be mad”, or Plato, “he who is master of himself has knocked in vain at the doors of poetry”; or Aristotle, “no great genius was without a mixture of insanity”; the mind cannot express anything lofty and above the ordinary unless inspired. When it despises the common and the customary, and with sacred inspiration rises higher, then at length it sings something grander than that which can come from mortal lips. It cannot attain anything sublime and lofty so long as it is sane: it must depart from the customary, swing itself aloft, take the bit in its teeth, carry away its rider and bear him to a height whither he would have feared to ascend alone.Seneca the Younger (c. 4 BC-AD 65) Roman statesman, philosopher, playwright [Lucius Annaeus Seneca]
Moral Essays, “On Tranquility of Mind [De Tranquillitate Animi],” 17.10 [tr. W. Langsdorf (1900)]
Full text. Source of this Aristotle quote.