What Dryden said was this, "Great wits are oft to madness near allied"; and that is true. It is the pure promptitude of the intellect that is in peril of a breakdown. Also people might remember of what sort of man Dryden was talking. He was not talking of any unworldly visionary like Vaughan or George Herbert. He was talking of a cynical man of the world, a sceptic, a diplomatist, a great practical politician. Such men are indeed to madness near allied. Their incessant calculation of their own brains and other people's brains is a dangerous trade. It is always perilous to the mind to reckon up the mind. A flippant person has asked why we say, "As mad as a hatter." A more flippant person might answer that a hatter is mad because he has to measure the human head. - G.K. Chesterton, Orthodoxy