A central goal of modern generative grammar has been to discover invariant properties of human languages that reflect 'the innate schematism of mind that is applied to the data of experience' and that 'might reasonably be attributed to the organism itself as its contribution to the task of the acquisition of knowledge'. Candidates for such invariances include the structure dependence of grammatical rules, and in particular, certain constraints on question formation. Various 'poverty of stimulus' (POS) arguments suggest that these invariances reflect an innate human endowment, as opposed to common experience: Such experience warrants selection of the grammars acquired only if humans assume, a priori, that selectable grammars respect substantive constraints. Recently, several researchers have tried to rebut these POS arguments. In response, we illustrate why POS arguments remain an important source of support for appeal to a priori structure-dependent constraints on the grammars that humans naturally acquire.