Radford (1988: 2) and others before and after have argued that the core theoretical question (i) ‘How are human languages structured?’ is logically prior to another core theoretical question (ii) ‘How are human languages acquired?’ This is said to be so because the answer to (i), the structure of human language, is the real target of question (ii) – language acquisition is about acquiring the structure of a human language. Thus, we can't ask about how a thing is learned before we know what it is. Bearing this dictum in mind, consideration of both Crain & Thornton (1998) (C&T) and the Drozd critique (present volume) raises an interesting question: is linguistic theory and the data on which it is established sufficiently developed to inform ‘external’ empirical studies in areas such as language acquisition? At this point, it isn't clear that it is. The data of theoretical linguistics is often idealized and sometimes rather narrow. A given construction may be narrowly investigated (cited) for its apparent interest as a side-light on another area or a more general theoretical issue rather than being investigated more fully/paradigmatically in its own right. Further, the latter sort of investigation might shed a very different light on a phenomenon only investigated narrowly. So paradoxically, it is often only in the context of doing experimental work such as acquisition or variation research that such fuller investigation takes place, possibly revealing new and crucial facts which may prove relevant to theory construction.