Popper had a fateful meeting with the philosopher of language Ludwig Wittgenstein at the Cambridge Philosophy Club. In a talk to the Club, with Wittgenstein in the audience, Popper described several “philosophical problems”– important, dif1⁄2cult questions that he thought would one day be answered. Here Popper was issuing a direct challenge to Wittgenstein, who had argued that philosophy could only analyze linguistic puzzles–not solve any real problems. The visit has become most famous for the subsequent controversy among eyewitnesses over whether or not Wittgenstein’s response to this challenge was to angrily brandish a 1⁄2replace poker at Popper. But there is a more interesting aspect to the story. One of the problems Popper described was the problem of causal induction: How is it possible for us to correctly infer the causal structure of the world from our limited and fragmentary experience? Popper claimed that this problem would one day be solved, and he turned out to be right. Surprisingly, at least part of the solution to the problem comes from a source about as far removed from the chilly Cambridge seminar room of 1⁄2fty years ago as possible–it comes from babies and young children. The past thirty years have been a golden age for the study of cognitive development. We’ve learned more about what babies and young children know, and when they know it, than we did in the preceding two thousand years. And this new science has completely overturned traditional ideas about what children are like. The conventional wisdom, from Locke to Freud and Piaget, had been that babies and young children are irrational, egocentric, pre-causal, and solipsistic, governed by sensation rather than reason, and impulse rather than intention. In contrast, the last thirty years of research have taught us that even the youngest infants–literally newborns –already know a great deal about a wide range of subjects. Moreover, we have been able to chart consistent changes