What is striking about human categories is their diversity. They range from the simplest classification of a face or color to the most carefully constructed taxonomic grouping. Considering this diversity, many are struck by the apparent gap between the simple, intuitive categories formed by children and the complex, theory-laden categories of educated adults (e.g., Inhelder & Piaget, 1964; Quine, 1969; Vygotsky, 1962).

In this chapter we first argue that despite a number of salient differences between children's categories and those of adults, there are important parallels between the two: Both are informed by an ability to overlook salient appearances, an attention to nonobvious properties, and the potential to draw many new inferences about the unknown. Both the initial groupings of the prescientific child and the most thoughtful, theory-laden classifications of the adult extend knowledge in important ways. To use Quine's terminology, both children and adults form “theoretical kinds.” Second, we address the role of language in the formation of theoretical kinds. Although the structure of everyday object categories (e.g., dog, hammer, oak tree, and computer) is traditionally thought to result from the structure of the world and/or the nature of human perception and cognition, we will present evidence that language is also critical and that how objects are named helps determine the structure of the categories they fall into.

The chapter has three sections. In the first, we set forth our assumptions about the nature of categories for adults. We review recent analyses suggesting that categories are enriched and informed by intuitive theories, summarizing arguments from psychology, philosophy, and linguistics to converge on the point that human categories extend far beyond observable similarities.