The Gem Merchant’s Eye

A gem merchant examined a stone. Before any test, he estimated from experience: seven in ten stones from this mine were flawed.

He held it to the light. No visible cracks. This updated his estimate — flawed stones sometimes hid cracks, but flawless stones rarely showed them. Now he thought perhaps four in ten that it was flawed.

He tested its weight. Slightly heavy for its size — a sign of density, suggesting fewer internal voids. He revised again: two in ten.

Each observation didn’t replace his previous belief. It refined it. His final judgment was not a guess. It was a calculation built on prior knowledge, updated step by step with each new piece of evidence.