monomaniac

— To talk nonsense, or poetry, or the dash between the two, in a tone of profound sincerity, and to enunciate solemn discordances with received opinion so seriously as to convey the impression of a spiritual insight, is the peculiar gift by which monomaniacs, having first persuaded themselves, contrive to influence their neighbours, and through them to make conquest of a good half of the world, for good or for ill.

— ...then it was that monomaniac Ahab, furious with this tantalizing vicinity of his foe, which placed him all alive and helpless in the very jaws he hated; frenzied with all this, he seized the long bone with his naked hands, and wildly strove to wrench it from its gripe.