In their first few years of life, children are making sense of the world at two levels at once: at the fine-grained level of everyday object categories (deciding which things are trees and which are dogs and which are cookies), and at a broader level that some have called commonsense “theories.” Both are remarkable achievements. First, consider categorization. If children's vocabulary is any indication, by the age of 6 they have carved up the world into thousands of distinct categories (Carey, 1978). Many children undergo a vocabulary “explosion” at roughly 18 months of age (Halliday, 1975; McShane, 1980; Nelson, 1973), when the rate of acquisition suddenly rises exponentially. One child studied in detail by Dromi (1987) produced as many as 44 new words in one week, and roughly 340 new words in her first 7 months of speech. No other species acquires symbolic communication at this rate. Even studies that successfully teach apes to acquire sizeable vocabularies in sign language are incomparable, with no noticeable vocabulary explosion (e.g., after more than 4 years of exposure to sign language, Washoe acquired only about 132 signs; Gardner & Gardner, 1989).

At around the same time that children learn to classify individual entities and undergo rapid vocabulary growth, they are developing broad systems of belief about the world. Not only do children learn to identify certain objects as “dogs,” but they also learn that dogs belong to the class of animals, and that animals engage in characteristic biological processes such as growth, inheritance, and self-generated movement. Children are learning about physical laws such as gravity, mental states such as dreams, and social relationships within units such as families.