I said before that if you've heard of one psychologist, it's going to be Sigmund Freud. If you've heard of two, it's probably going to be Sigmund Freud and BF Skinner. But if you've heard of developmental psychologists, it's probably going to be Jean Piaget. Jean Piaget was the founder of the modern study of developmental psychology. A lot of the methods and observations and rich theories that we now build from came out from his work. He had a vibrant school. He had many students, and he had a really interesting influence on our field. And his research program was genetic epistemology. And one way to put it, which I think is admittedly too crude, but we'll try this out, is that Jean Piaget ultimately wasn't interested that much in children. Rather, he was interested in the development of knowledge in the human species. And the way he pursued that interest, though, because you can't study the human species, it's not in front of you, you have to infer it through other sources, is to look at it in individual children. The idea that John Piaget believed is that, well, to put it in a very snooty phrase, ontogeny recapitulates phylogeny. And you can forget the phrase, but what the idea would mean is that you can see the development of the species is repeated, recapitulated, in the development of every individual. So what did Jean Piaget have to say? Well, he thought of children as active thinkers. He may call them little scientists trying to make sense of the world. And he described these understandings as schemas. And schemas were mental systems. So you could already see that although Piaget was in some way had views congenial to those of empiricists with his emphasis on learning, he is already departing from them in his believe that there could be mental structures in the head. And these schemas were frameworks that we build upon to acquire knowledge. And for Piaget, there were two psychological mechanisms that lead to the transformation of these schemas. And to sometimes to the creation of new schemas. One is assimilation. So assimilation is the process of taking in new information and new experiences and matching it up with an already existing schema. So imagine a baby who already knows how to suck on his mother's breast, but now the baby sucks on a rattle or sucks on his own toes. And then there's accommodation. And accommodation is the process by which existing schemas are changed, or new schemas are created, in order to fit the new information and new experience. So a child who has to suck on his rattle or his toes is going to do so in a somewhat different way than sucking on his mother's breast. And that's a very concrete, maybe not so interesting example. But Piaget argued that this held, too, for more abstract and intellectual attainment. So that there are schemas having to do with objects or number or people. And the baby's understanding, ultimately the child's understanding, get transformed as a result of the process of assimilation and accommodation. Now, Piaget's method was to ask children, was to talk to children. He has wonderful interviews with children where he would ask them to solve problems, or deal with different situations. And then, question them thoroughly, intensively, about the reasoning behind their solutions. And this is an interesting view, and a radical view, that children think in entirely different ways than adults. So he's not like a nativist suggesting that children know it all from the start. But nor is he like an empiricist or a behaviorist who viewed learning simply as the accumulation of new knowledge. Or for the behaviorist, new conditioning and new responses. Rather, Piaget believed that children have theories of the world, and so do we. And children's theories are very different from ours. And so, he proposed that theories transform, and that you could capture development in terms of a series of stages. Each stage corresponding to a different style of thinking, a different way of making sense of how the world works. And so, we've already seen a stage theory with Freud, talking about anal stage and oral stage and so on. But Freud theory was psychosexual. Piaget's theory is sort of far more intellectual, far more having to do with styles of reasoning. And in my view, has far more scientific grounding.