Specialists in genetic psychology, and especially in child psychology, do not always suspect what diverse and fruitful relationships are possible between their own subject and other more general kinds of research, such as the theory of knowledge or epistemology. And the converse is even more true, if that is possible: that child psychology has for long been regarded as a collection of case histories of infants. The necessity has not always been recognised, even in the field of general psychology, of considering all problems from the standpoint of development, and it is still true, in certain countries, that ‘child psychologists’ are a group apart, having no contact with the main currents of experimental psychology. Even less, as a rule, do students of the theory of knowledge suspect that, within reach as it were, in the field of psychogenetic experience, they can sometimes find solutions to the most general questions on the formation of ideas or on the analysis of intellectual activity. Yet they have been known to show inexhaustible patience when trying to reconstruct some unknown passage from the history of science for the sake of its epistemological significance.