The first nine months of ontogenetic development was once considered as a high-speed rerun of phylogenetic development. For example, a fin-like structure in the human embryo was seen as representing our past in the sea. Ontogeny recapitulates phylogeny. Embryologists and paleontologists worked for half a century within this hypothesis, but the case for the theory of recapitulation was finally dismissed for lack of evidence.

There may, however, be certain formal similarities between ontogenetic and phylogenetic development. Both processes are characterized by continuous discontinuity. Charles Darwin argues that phylogenetic development is continuous with respect to function (organisms evolve through adaptation to the environment) but is discontinuous with respect to structure (different organisms evolve to fit different environments). Jean Piaget, like Darwin, views the function of the nervous system as the adaptation of the organism to the environment. However, he goes further by arguing that adaptation involves the alternating processes of assimilation (in which the organism takes in information from the environment) and accommodation (in which the organism adjusts its cognitive structure, if necessary, to assimilate this information). Thus, ontogenetic development is continuous with respect to function (the person adapts to the environment through alternating assimilations and accommodations) but discontinuous with respect to structure (different structures emerge over time to accommodate what is assimilated).

Piaget had to convince us of the discontinuity, since he had to fight the prejudice that a child is a miniature adult; Darwin had to convince us of the continuity, since he had to fight the prejudice that the human organism is unique. Piaget and Darwin have matured and humanized we adult humans by, paradoxically, showing us our affinity with children and with animals.

Jean Piaget was a precocious child. He published his first scientific paper at 10, refused an important position as curator of a museum at 14, and completed his doctorate at 22. Unlike many such fast starters, Piaget continued to outpace his peers and his times and was still precocious when he died in his eighties. No matter how precocious, however, he still went through the same stages of cognitive development in the same order as you and I. This is his theory.

Though Piaget was mainly interested in epistemology (the study of knowledge), he took his formal training in biology and, though he was trained in biology, he devoted his life's work to developmental psychology. It is career choices like this which drive guidance counsellors crazy. Superficially, his decisions could be attributed to chance. Visits to the Swiss countryside during his childhood triggered an interest in the flora and fauna around him. Hence the training in biology. A position in Binet's laboratory designing items for intelligence tests got him interested in the children's wrong answers. Hence the career in developmental psychology. However, Piaget's career was more consistent than it seems. Developmental psychology (the subject to which he devoted his life) is the missing link between biology (the subject in which he was trained) and epistemology (the subject in which he was primarily interested). That is, the link between the function of the nervous system (biology) and the content of the nervous system (epistemology) is the evolving structure of the nervous system (developmental psychology).

The theory of Jean Piaget may be as important as those of Charles Darwin and of Sigmund Freud. Whereas Darwin replaced the animal-human dichotomy with a dimension and Freud replaced the normal-abnormal dichotomy with a dimension, Piaget replaced the child-adult dichotomy with a dimension. It is interesting to note that those three who made revolutions did so by destroying, rather than by creating, dichotomies. However, though the Piagetian revolution is probably as important as the Darwinian revolution and the Freudian revolution, it is not as familiar.^₁

It is not so familiar for a number of reasons. First, it is newer. A new theory tends to be greeted initially as preposterous ("What!") and eventually as obvious ("So what?"). The theories of Darwin and Freud have been around long enough to become obvious, whereas the theory of Piaget, while no longer preposterous, is not yet obvious. Second, it is difficult to understand. A new theory requires a new language. Piaget has developed an elaborate and idiosyncratic system of technical terms to state his theory. The usual difficulty of translating from French to English is compounded by the additional difficulty of translating from Piageteze to French. Third, it runs counter to the prevailing current. Piaget's method is uncongenial and his theory is unpalatable to the behaviorists who have dominated psychology.

However, Piaget's theory has recently, despite those disadvantages, become well-known to psychologists and educators. Indeed (perhaps, to the detriment of the theory and the embarrassment of the theorist) it has even become fashionable. Since the methods and theories of Darwin and Freud are more familiar, however, I will present those of Piaget by comparison with them. We will see that the method of Piaget is similar to that of Freud and that the theory of Piaget is similar to that of Darwin.