Some aspects of a theory of grammar are presented which derive from a formal theory of language acquisition. One aspect of the theory is a universal constraint on analyzability known as the Freezing Principle, which supplants a variety of constraints proposed in the literature. A second aspect of the theory is the Invariance Principle, a constraint on the relationship between semantic and syntactic structure that makes verifiable predictions of syntactic universals. The relationship between the notion of "explanatory adequacy" of a theory of grammar and the learnability of a class of transformational grammars is discussed. (Author)