From Adam Gopnik’s “The Porcupine: A pilgrimage to Popper”:

In the real world, as Popper knew perfectly well, the response of the scientist who has proposed that all swans are white when a black swan appears is not to say, cheerfully, “Wrong again!” It is to say, “You call that a swan?” The principle of falsification would begin an argument rather than prove a point. But the argument was the point. The argument that the black swan would produce—an argument about what evidence was crucial, and why—was different from all other kinds of argument. Science wasn’t a form of proof. It was a style of quarreling. The reason science gave you sure knowledge you could count on was that it wasn’t sure and you couldn’t count on it. Science wasn’t the name for knowledge that had been proved true; it was the name for guesses that could be proved false.