Alexis de Tocqueville:

submitted to complete isolation; but this absolute solitude, if nothing interrupts it, is beyond the strength of man; it destroys the criminal without intermission and without pity; it does not reform, it kills

Victor Hugo:

The entirety of hell is contained in one word: solitude.

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Ippolit Terentyev

Oh, you may be sure that Columbus was happy not when he had discovered America, but when he was discovering it; you may be sure that the highest moment of his happiness was, perhaps, exactly three days before the discovery of the New World, when the mutinous crew in their despair almost the ship back to Europe, right around! The New World is not the point here, it can just as well perish … The point is in life, in life alone – in discovering it, constantly and eternally, and not at all in the discovery itself!