“Beware of bugs in the above code; I have only proved it correct, not tried it.” That was how Donald Knuth, author of The Art of Computer Programming (1968), expressed the difference between pristine mathematics and buggy reality. “When programming, you abstract away the entire physical world as much as possible, because it’s messy. But then it comes back and bites you,” Paul Ford, cofounder of the platform-builder Postlight, told me.

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