In short, the twin assumptions that syntactic rules are category-based, and that there are a highly restricted finite set of categories in any natural language (perhaps no more than a dozen major categories), together with the assumption that the child either knows (innately) or learns (by experience) that all rules are structure-dependent ( =category-based), provide a highly plausible model of language acquisition, in which languages become learnable in a relatively short, finite period of time (a few years).