This volume examines some of the arguments that have been put forward over the years to explain the way in which understanding is acquired. The author looks firstly at the empricist thesis of genesis without structure, and secondly at the opposing theory, represented by Chomsky of structure without genesis. His greatest sympathy is with the theory of Piaget, who represents structure with genesis. He considers that Piaget's account is flawed, however, by its biological model and by its failure to deal adequately with the problem of objectivity. The second part of the book contains chapters on language, the differences between early and later learning, and on teaching. The book provides a general understanding of the principles that make it possible, and the differences between the ways in which they work at different stages.