I defend the non-cognitivist outlook on knowledge of grammar from the criticisms levelled against it by Jonathan Knowles. The first part of the paper is largely critical. First, I argue that Knowles's argument against Christopher Peacocke and Martin Davies's non-cognitivist account of the psychological reality of grammar fails, and thus that no reason has been given to think that cognitivism is integral to an understanding of Chomskyan theoretical linguistics. Second, I argue that cognitivism is philosophically problematic. In particular, I argue (...) (a) that Knowles misunderstands Stephen Stich and Gareth Evans's points about inferential integration; and (b) that Knowles misunderstands the philosophical status and demands of Evans's Generality Constraint. In the final, non-critical part of the paper, I try to show that the Stich and Evans's constraints, together with a Self-Knowledge Constraint, are genuine constitutive constraints by showing how they have rationality as their underlying organizing principle. I argue further that recognizing the constitutive character of these constraints allows one to distinguish two kinds of psychological explanation; in this sense, the constraints mark a genuine boundary in our theorizing about the mind. (shrink)