“Madness is again and again the result of perfectly logical reasoning upon a wrong premise.”G. K. Chesterton · Ortodoxia · 1908 · capitolul 2
Madness is not a lack of logic, but flawless logic built on a false premise.
Chesterton notes that the madman does not think chaotically — on the contrary, he builds an impeccable system around one false idea, say, that all policemen are against him. Every fact is interpreted to confirm the premise. The mechanism is that errors are not corrected inside a closed system; only doubt about premises does that. It is like a computer that computes perfectly but receives wrong input data.
Why it matters It warns against any unshakeable conviction, personal or public: if no evidence can contradict you, the problem is not your logic but your starting point.