Introduction

We compare objects to each other in a variety of ways. We experience our world in terms of a complex system of distinct kinds of perceptual similarities. We judge objects to be similar or different. We also judge objects to be similar and different in part – to be, for example, similar in color and different in size. We categorize objects by their attributes and in so doing judge them to be similar; for example, we categorize objects as red, as blue, as big, as small. We compare objects in terms of their direction of difference – judging, for example, one object to be smaller than another. This variety of kinds of judgments clearly indicates that perceptual similarity is not one thing but is of many interrelated kinds. In brief, we seem to possess a complex system of perceptual relations, a complex system of kinds of similarity. The concern of this chapter is with the development of a system of knowledge about such relations.

The evidence suggests that an understanding of perceptual relations develops quite slowly during the preschool years. Indeed, working out a system of perceptual dimensions, a system of kinds of similarities, may be one of the major intellectual achievements of early childhood. The evidence for an emerging dimensional competence is widespread – and includes developments in Piagetian conservation tasks (e.g., Piaget, 1929), in seriation and transitive inference tasks, in classification tasks (e.g., Inhelder & Piaget, 1958, 1964), in transposition learning (e.g., Keunne, 1946) and discriminative learning tasks (e.g., Kendler, 1979).