If any human acts may loosely be called causeless, they are the minor acts of a healthy man; whistling as he walks; slashing the grass with a stick; kicking his heels or rubbing his hands. It is the happy man who does the useless things; the sick man is not strong enough to be idle. It is exactly such careless and causeless actions that the madman could never understand; for the madman (like the determinist) generally sees too much cause in everything. The madman would read a conspiratorial significance into those empty activities. He would think that the lopping of the grass was an attack on private property. He would think that the kicking of the heels was a signal to an accomplice. If the madman could for an instant become careless, he would become sane. Every one who has had the misfortune to talk with people in the heart or on the edge of mental disorder, knows that their most sinister quality is a horrible clarity of detail; a connecting of one thing with another in a map more elaborate than a maze. If you argue with a madman, it is extremely probable that you will get the worst of it; for in many ways his mind moves all the quicker for not being delayed by the things that go with good judgment. He is not hampered by a sense of humour or by charity, or by the dumb certainties of experience. He is the more logical for losing certain sane affections. Indeed, the common phrase for insanity is in this respect a misleading one. 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, from Orthodoxy