Every structure is to be thought of as a particular form of equilibrium, more or less stable within its restricted field and losing its stability on … - Jean Piaget

Formal logic, or logistics, is simply the axiomatics of states of equilibrium of thought, and the positive science corresponding to this axiomatics is none other than the psychology of thought.

The discussion of the game of marbles seems to have led us into rather deep waters. But in the eyes of children the history of the game of marbles has quite as much importance as the history of religion or of forms of government. It Is a history, moreover, that is magnificently spontaneous; and it was therefore perhaps not entirely useless to seek to throw light on the child's judgment of moral value by a preliminary study of the social behaviour of children amongst themselves.

I … sent a program of research to the Rockefeller Foundation. … [Their] questions were almost all of a surprising pertinence. I remember the practical questions … But I particularly remember the theoretical questions, due among others to [Shannon] Weaver, the mathematician interested in information theory who was then in charge of the Department of Science at the Foundation: How will you find interesting epistomological ideas, for example, the theory of relativity, in studying children who know nothing and who in any case are brought up in the intellectual tradition dating from Newton? …

I had the luck to be able to remark … that Einstein himself had advised me in 1928 to study the formation of the intuitions of velocity in order to see if they depended on those of duration, and that further, when I had the good fortune to see Einstein again at Princeton …, he was quite delighted by the reactions of nonconservation of children of four to six years (they deny that a liquid conserves its quantity when it is poured from one glass into another of a different shape: ‘There is more to drink than before,‘ etc.), and was greatly astonished that the elementary concepts of conservation were only constructed toward seven or eight years.