“The man who believes in himself is almost sure to be a madman; the man who believes in a creed is almost sure to be a sane man.”G. K. Chesterton · Ortodoxia · 1908 · capitolul 3
A mind that measures only itself has nothing to correct its errors.
Chesterton draws a counterintuitive line: if your own head is the only measure of truth, you have nothing to correct yourself against, so every error grows unchecked. Someone who accepts a inherited creed, by contrast, has an outside reference tested across generations. A man certain he is always right and deaf to everyone drifts into conspiracies; a man anchored in a creed always has something to weigh his judgments against.
Why it matters It lands in an age where everyone builds a private bubble of truth online. The line warns that unbounded confidence in your own mind, with no outside reference, is a reliable recipe for drifting off course.