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		given collections of languages are learnable by children. How can this information be acquired? Direct experimental approaches are ruled out for obvious reasons. Investigation of existing languages is indispensable, since such languages have already been shown to be learnable by children; as revealed by recent studies (e.g., van Riemskijk and Williams [189] and Werker [193]), much knowledge can be gained by examining even a modest number of languages. We might hope for additional information about learnable languages from the study of children acquiring a first language. Indeed, many relevant findings have emerged from child language research. For example a child's linguistic environment appears to be devoid of explicit information about the nonsentences of her language (see Section 3.2, below). As another example, the rules in a child's immature grammar are not simply a subset of the rules of the adult grammar but appear instead to incorporate distinctive rules that will be abandoned later (see Pinker [148]).

		However, such findings do not directly condition theories of comparative grammar. They do not by themselves reveal whether some particular class of languages is accessible to children or whether it lies beyond the limits of their learning. The theory of empirical discovery described in this book may be conceived, in part, as an attempt to provide the inferential link between the results of acquisitional studies and theories of comparative grammar. It undertakes to translate empirical findings about language acquisition into information about the kinds of languages accessible to young children. Such information can in turn be used to evaluate theories of comparative grammar.

		To fulfill its inferential role, the theory of empirical discovery offers a range of models of language acquisition. This is achieved by providing precise construal of concepts generally left informal in studies of child language, namely, the five concepts listed in Section 1.2. The interesting paradigms from the point of view of comparative grammar are those that best represent the circumstances of actual linguistic development in children. The deductive consequences of such models yield information about the class of possible natural languages. Of course, the paradigm to be presented in this chapter is only a crude representation of language acquisition by children. Many of the models to be discussed later are attempts to improve its fidelity.

		More extensive discussion of the role of paradigms in comparative grammar is available in Wexler and Culicover [194], in Osherson, Stob, and Weinstein [138], in Osherson and Weinstein [144], and in Osherson, Weinstein, de Jongh, and Martin [145]. We shall return frequently to both language acquisition and scientific discovery in order to motivate our theory.