The madman is not the man who has lost his reason. The madman is the man who has lost everything except his reason.

-- G.K. Chesterton, quoted in Jonah Lehrer's How we decide

(In the section which discusses psychopaths and notes that the "rational" part of their brains appears to be undamaged: the human brain relies on the circuitry of emotion to form moral decisions, or at any rate that's what's broken in psychopaths.)