Einstein once praised the Swiss psychologist Jean Piaget for having had the audacity of discover a most elementary insight: that children think differently than adults do. They are not just immature adults, but have their own distinctive way of approaching the world that can yield to the sedulous investigations of the psychologist, if he is imaginative enough to forget his own accustomed manner of thought and to observe children with an attentive, open mind. No doubt, Piaget excelled at this very thing and, what is more, had the patience and determination to see his investigations through to a grand synthetic conclusion, which he has outlined in several of his works.
The present volume, The Child’s Conception of Number, turns Piaget’s fruitful approach to the origin and derivation of mathematical concepts, in particular, number and arithmetic. Now a child of three years can count, of course, but it does not mean, by a long shot, that he has a real conception of the numbers he enumerates, as opposed merely to having memorized a sequence of names (with certain built-in patterns, which vary from language to language). In fact, Piaget concludes from his researches that children do not arrive at an adequate conception of what a number is until as late as the age of eight. In the work under review, he reconstructs the stages through which children typically go, at which he has arrived through induction based on experimental study of young children over a range of ages. His technique is to interview the child and to ask him questions about toys provided for the purpose (of course, this presupposes the child to be old enough to have command of speech; earlier stages can be postulated hypothetically). The written record of these interviews can be sorted, organized and analyzed in order to infer what the child at a given age is actually capable of understanding. A nice feature of the present work is that the author has reproduced a representative selection of these interviews, so that the reader can retrace Piaget’s argument from the children’s very words themselves. The investigations recounted here may be seen as an application of Piaget’s general theory of intelligence (cf. his foundational Psychology of Intelligence), where the author’s views on accommodation, assimilation and the operationalist approach to intelligence come to the fore.
Piaget summarizes his findings in compressed form in the Foreward: ‘Our hypothesis is that the construction of number goes hand-in-hand with the development of logic, and that a pre-numerical period corresponds to the pre-logical level. Our results do, in fact, show that number is organized, stage after stage, in close connection with the gradual elaboration of systems of asymmetrical relations (qualitative seriations), the sequence of numbers thus resulting from an operational synthesis of classification and seriation. In our view, the logical and arithmetical operations therefore constitute a single system that is psychologically natural, the second resulting from generalization and fusion of the first, under the two complementary headings of inclusion of classes and seriation of relations, quality being disregarded.’
Let us unpack what he says here. The point is that, to begin with, the child has no concept of quantity at all. At the early, pre-numerical stage, all he knows is how to perform qualitative comparisons of two things presented immediately in his visual field: the level of water in this glass is higher than that in that glass etc. A lot of conceptual work is needed before the child can begin to form an adequate concept of number. Piaget conveniently demarcates the properties of number into two: ordinality and cardinality. Ordinality, as the term suggests, has to do with the property of the real line of being an ordered field: given two numbers, we can say whether one is greater than, less than or equal to the other. The child has to acquire the concept of ordinality gradually by becoming proficient at seriation, by which Piaget means to place a collection of various-sized items into sequential order. The youngest children cannot do this at all, even when the procedure is demonstrated to them before their eyes. A number of concepts have to be mastered first, such as that of conservation and one-to-one correspondence. At four to five years, for instance, a child will be convinced, when one pours liquid from a wide glass into a narrow glass, that the higher level in the latter means that the quantity of water has increased. Likewise, given a collection of pennies placed on a table, he will judge their number changes when one permutes them. Only when he recounts them by hand will he believe that the number has stayed the same.
There is no need to reconstruct Piaget’s entire argument here. The reader will have the pleasure of watching how Piaget proceeds step by step to recover the development by which the ingredients of the concept of number are built up in the child, basing himself everywhere on the children’s self-reports during his experiments. The critical stage begins at seven to eight years, when the child masters concrete operations and applies the understanding thereby gained to the concept of number. The child learns to seriate and to perform reversible operations, such as permuting a set of objects and seeing that their number is conserved. The final stage in acquiring an adequate concept of number is the mastery of elementary arithmetical operations of addition, subtraction and multiplication. As Piaget relates, a lot has to go on here behind the scenes, such as the comprehension of division into equal parts and the composition of parts back into the whole. To compare two sets of objects for their cardinality, mentally one abstracts units one at a time, where it has to be understood that the total quantity in each set remains invariant during this process. The mature concept of number emerges as the conviction of reversibility of operations grows and the child begins to coordinate operations into a coherent unity.
We leave it to the reader to follow the flow of Piaget’s logic in detail and append some reflections on the significance of what Piaget has accomplished in this work. These reflections have a bearing on the controversy between the analytic (Frege, Russell) versus synthetic (Kant) philosophies of mathematics. In the view of Russell’s logical formalism, arithmetic involves nothing but concepts subject to logical operations, while for Kant, intuition (Anschauung) has to intervene, for to form the sum of two natural numbers is to construct (in the imagination) the series of successive units until one arrives at their sum, somewhat as one draws lines or circles in the course of a geometrical demonstration, which for Kant, of course, involves spatial intuition (outer sense), as against the temporal intuition (inner sense) underlying arithmetic. For this reviewer, what Piaget has done is to show that number is derived from a synthesis of perception and intelligence applied to operation. As such, the axioms of arithmetic represent a condensation of thought; we could not have arrived at them without first carrying out the synthesis in intuition.
The preceding observation invites another speculation: perhaps we could go back a step and rederive another larger concept of number than the standard one. The numbers with which we are familiar are useful to us because of the interaction with the world made possible by calculation by means of them, but that does not necessarily mean that we have exhausted, up to now, the range of possibilities. What the reviewer has in mind is something like a decategorification, as it is understood in modern mathematics. Indeed, it is possible that we could apply to number something analogous to the idea of equivalence up to homotopy in higher category theory. Just a suggestion for enlarging and enriching our experience of the world and corresponding mathematical formalism! Then, what Russell regards as analytic truths about number could be relative only to a certain formalization of them, just as Kant falsely imagines Euclidean geometry to be analytically true of spatial intuition. Now, all we need is for a young genius to discover the non-Peano arithmetic that would parallel non-Euclidean geometry!
P.S. For a graphical illustration of what the last-mentioned point could imply, consult the not-otherwise-distinguished Hollywood movie from a few years ago, Lucy, which is notable for the implicit philosophy of mathematics it espouses. This reviewer would like to contradict those who criticize the movie for being mathematically unrealistic, not to say impossible.