THE standard joke about artificial intelligence (AI) is that, like nuclear fusion, it has been the future for more than half a century now. In 1958 the New York Times reported that the “Perceptron”, an early AI machine developed at Cornell University with military money, was “the embryo of an electronic computer that [the American Navy] expects will be able to walk, talk, see, write, reproduce itself and be conscious of its existence.” Five decades later self-aware battleships remain conspicuous by their absence. Yet alongside the hype, there has been spectacular progress: computers are now better than any human at chess, for instance. Computers can process human speech and read even messy handwriting. To cynical moderns, automated telephone-answering systems are infuriating. They would seem like magic to someone from the 1950s. These days AI is in the news again, for there has been impressive progress in the past few years in a particular subfield of AI called machine learning. But what exactly is that?