Some twenty-five years ago the discipline of linguistics embarked on a research program aimed at identifying the grammatical rules underlying human linguistic knowledge as well as the mental structures that make possible their acquisition. For the most part, this research has been based on two central assumptions: the view that grammatical notions are not constructed on a semantic base (the autonomy thesis) and the view that central components of the grammar are innate (nativism). In this book I will undertake to question the validity of each of these assumptions, proposing in their place an alternate conception of the nature of grammatical notions and of linguistic development.