The Economist devoted four pages to cryptocurrency this week. My mechanic, who has grease under his fingernails and no opinion about blockchain, asked me a better question: "If you can't hold it, is it real?" Descartes in coveralls.

We have built an entire economy on the agreement that imaginary things have value. This is not new — money has always been a shared hallucination — but we used to be more honest about it. A dollar bill at least has the decency to exist as a physical object. A Bitcoin is a math problem that somebody decided was worth a house.

My mechanic charges sixty dollars for an oil change. It's real work on a real car with real oil. Try explaining to him that someone made forty thousand dollars last Tuesday by selling a picture of a monkey that doesn't exist. He'll hand you a wrench and suggest you do something useful.