Jean Piaget, the father of child psychology, was the first great thinker of the 20th century to take children's thoughts seriously - where others saw sweetly lisping innocents, he saw young scientific explorers. His work - which is celebrated at an international conference on The Growing Mind in Geneva this weekend - spanned psychology, philosophy, pedagogy and biology. Out of this potent brew he fashioned a new academic area, which he called genetic epistemology: the study of how we come to know what we know.
In dozens of books on the cognitive and moral development of children he tried to trace the structures underlying human intellect as it evolves from babyhood to maturity. Uniquely, he founded his work on the close observation of his own children, forming fertile hypotheses from pondering the significance of childish endeavours and, most importantly, the meaning of their mistakes.
Piaget's work, dense and abstruse as it mostly is, hit a nerve in the postwar Zeitgeist and was widely employed in teacher training and curriculum development in Britain from the early Sixties. During the Eighties, however, the politics of education shifted and Piaget, who had been conscripted into the "child-centred" approach advocated in the 1967 Plowden Report, became unfashionable. Today, changes in teacher training mean that precious few student teachers read much about the theory of education; changes in the curriculum mean that less thought is given to adapting what is taught to how children at different stages may learn it.
None the less, as Harry Judge, former Director of the Department of Education at Oxford University, puts it, Piaget "has become part of the air we breathe".
A child prodigy who published his first academic paper at the age of 10 (on the albino sparrow), Jean Piaget was born and educated in Switzerland, where he spent most of his life. By the age of 15 he was a respected zoologist and an authority on molluscs. By the time he went to university he had widened his interests to include philosophy and, after a doctorate in zoology, switched from molluscs to mental capacity, studying psychology in Zurich with Carl Jung and Bleuler (authorities respectively on psychoanalysis and psychiatry) and in Paris with Alfred Binet (who invented intelligence testing).
Working with Binet, Piaget gave reading tests to young children and gradually became interested in what their mistakes revealed about how their minds worked. In 1921 he began publishing books on children's thinking and was appointed director of the Institut Jean-Jacques Rousseau in Geneva. Professor by 1926, writer of more than 50 books and monographs, founder of the Centre of Genetic Epistemology in 1955, he remained an active and encyclopaedic thinker until the end of his long life in 1980. Titles like Judgement and Reasoning in the Child (1924), The Origins of Intelligence in Children (1948), The Moral Judgement of the Child (1932) and The Child's Conception of Space (1956) give a flavour of how he systematised the study of human cognitive development.
He was also a dedicated family man and fond parent to his three children, whose early lives offered fruitful seedbeds for his research.
Piaget saw intelligence in biological terms, as a question of an organism adapting to its environment and tending always to a state of renewed equilibrium. Children think about the world and change their ideas about it as they adapt to it. Structurally, there is always a pattern which produces progressively more stable states of adaptation, which Piaget identified as stages.
There are four basic stages and some sub-stages. At first, up to 18 months or so, there is the sensori-motor stage, in which the world is understood through bodily processes. Then comes the pre-conceptual stage, until about four or five, in which the child begins to use symbols, and hence language, but cannot generalise from personal experience. This egocentric stage gives way slowly to the "concrete operational stage" where the child can reliably manipulate classifications. For instance, a child by the end of this stage (from about seven to 11) should be able consistently to understand that a bowl of different coloured beads can seen as overlapping sets of different colour, sets of different size, shape or material - and yet remain one bowlful. Then comes the "formal operational" stage, perfected by about 15, where abstract concepts like heat or light can be managed without reference to any concrete objects.
Piaget himself never tied these stages inflexibly to any age and in later years also conceded that some individuals might progress unevenly through the stages in different areas - for instance, by becoming visual artists who could not do sums. Piaget did, however, insist that the order of stages never varied and that, as the progress they described was hierarchical, it was impossible to miss out any stage.
Piaget has been criticised and challenged over the years, though often by people who have never read or digested his juggernaut tomes. As Ted Wragg, Professor of Education at the University of Exeter, says, he is "a monumental figure, much misquoted and much misused". At the height of his popularity, in the Seventies, he was, remembers a student at the time, "the shining Messiah who inspired a new way of teaching which would overturn the old board school methods".
Yet, as Mary Jane Drummond at the Cambridge University Department of Education remarks, his work was over-simplified. He himself was not, until his last years, particularly interested in education and was not in the business of producing techniques for helping children learn.
As the first academic to observe and study children's thinking, to realise that learning is an active process and to try and describe that process, Piaget stands as a giant in the field. Other thinkers, notably Vygotsky, described parts of the process of development left out by Piaget, notably the place of language and social interaction. Bruner looked at the effect of what is taught on how children learn.
Then there are the worldwide cultural differences, ignored by Piaget's fairly uniform and small Swiss sample. And, though professing to be interested in emotions, Piaget preferred to look at the evolution of the kinds of mental constructs, such as those about mathematics and the physical world, that can be measured. These tended to back up his claims about the ways in which such understandings are absorbed. As Harry Judge says, his impact on the teaching of the arts and humanities is negligible.
On primary teaching, however, his influence was tidal. As Colin Richards, Visiting Professor of Education at the University of Leicester, says, his writings tapped in to the kind of progressive teaching which, thanks to the influence of educationalists such as Friedrich Froebel and Johann Pestalozzi, was already current in the infant schools of postwar Britain.
Activity and discovery-based teaching had already been seen to stimulate young children; Piaget's ideas offered a theory of how and why some things would work and others wouldn't. By replacing the idea of age-appropriate learning with stage-related learning, Piaget liberated teachers from the idea that children had to achieve a fixed level by a certain age, says Jo Goodall, field officer for the British Association for Early Childhood Education (BAECE).
If some teachers, as Ted Wragg laments, used this notion of each child progressing at his or her own pace as a "recipe for low expectations" - and the Plowden Report seemed to elevate child-centredness above achievement - others were spurred by Piaget to see the differences in potential for each individual. Piaget, above all, says Mrs Goodall, made teachers understand the importance of observing what is happening to each child at the earliest stages of cognition.
But where does this kind of stage-related teaching stand now? David Woolf, headteacher of Snape Primary School in Suffolk, notices that the student teacher who recently trained at his school knew nothing of Piaget - and that the great man's name featured as only one of many items in the bibliography, a "bolt-on bit of psychology".
Ted Wragg points, with regret, to the lack of time for reflection on issues, let alone on individual thinkers, in teacher-training timetables. Colin Richards worries lest "teachers are in danger of becoming skilled technicians implementing a curriculum devised elsewhere and with which they have no interaction". Without the beacons of past theorists to illuminate the stage, will teachers just be whistling in the dark? Mary Jane Drummond says, "We can't do without Piaget and all those giants."
At the chalkface, however, teachers like David Woolf say they would use more discovery-based methods, particularly in primary science, if only the national curriculum allowed enough time. Which leads on to Ted Wragg's question: "If the child is not at the centre of education, what is?" If the curriculum is at the centre and the child is not an active learner, how does teaching work?
Mary Jane Drummond is firm that "we can't teach the curriculum if we don't spend as much time thinking about learning as thinking about teaching". Whatever the current ideology, teachers have to take account of how children learn. They cannot help but be struck as much by their mistakes as by the sudden leaps they make in understanding. There is still much to learn.