The Burden of Proof Is Not Symmetric

Bertrand Russell was asked in 1952 to write about whether God exists, and he built his answer around a deliberately absurd example. Suppose, he said, I claimed that a china teapot orbits the sun somewhere between Earth and Mars, too small to be seen by any telescope. Nobody could disprove it. But, Russell argued, if he insisted that his claim therefore could not be doubted, this would rightly be considered nonsense — 'the burden of proof lies upon me' precisely because he made the assertion. His point was that unfalsifiable claims don't gain credibility from being unfalsifiable; they lose it. Carl Sagan later sharpened the principle into the line most people know: 'extraordinary claims require extraordinary evidence.' Sagan's version adds the crucial scaling idea — the strength of evidence required isn't fixed but rises with how much the claim conflicts with what's already well established.