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The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight.

“It’s not clear whether the original has become lighter, or the national prototypes have become heavier,” said Michael Borys, a senior researcher with Germany’s national measures institute in Braunschweig. “But by definition, only the original represents exactly a kilogram.”

Yes, but what would a philosopher say?