A clockmaker had two clocks on her shelf. One she'd built with a jeweled escapement, balanced wheel, temperature-compensated spring. It kept time within a second per week. The other had a cracked mainspring she hadn't yet replaced. It lost twelve hours a day — which meant it showed the correct time twice every twenty-four hours.
A customer came in at noon. Both clocks read 12:00.
"Which is the better clock?" he asked.
The clockmaker said, "At this moment, you can't tell."
He came back the next day at noon. Both read 12:00 again.
"So they're equally reliable," he said.
The clockmaker shook her head. "One arrived at noon by traveling through every minute between yesterday's noon and today's. The other arrived at noon by accident — the hands swept through twelve-o'clock because they were always sweeping, and twice a day the sweep coincides with the truth. If you came at 3:15, or 9:47, or any arbitrary moment, one clock would be right and the other would be wherever the broken spring left it."
"But I keep coming at noon."
"Yes," she said. "And if you only ever come at noon, you will never see the difference. The difference is in the hours you don't check."
He thought about this. "What if I need a clock only for noon?"
It was a fair question. She considered it honestly.
"Then the broken clock is sufficient. But you should know what you're relying on — not a clock that knows what time it is, but a coincidence between a broken mechanism and a particular question. If you ever need it for a different question, the answer will be wrong, and the clock won't know it. The working clock will be right at 3:15 and 9:47 and every moment you haven't thought to ask about. The broken one will be right only when your question happens to match its accident."
He bought the working clock. Not because he needed 3:15. Because he realized he didn't want to build his life around only asking questions he already knew the answer to.
The broken clock stayed on the shelf. Twice a day it was exactly right, and it never knew the difference.
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