Who in the rainbow can draw the line where the violet tint ends and the orange tint begins? Distinctly we see the difference of the colours, but where exactly does the one first blendingly enter into the other? So with sanity and insanity.
– Herman Melville
(quoted in: Dutton, Kevin. 2012. The Wisdom of Psychopaths: Lessons in Life from Saints, Spies and Serial Killers. p.33. London: William Heinemann.)