Ann Peters' interesting paper raises a number of questions in my mind. One of these arises from ‘the assumption that at any given point in development, children either do or do not ‘have’ the functional categories that underlie syntax.’ This dichotomy may be too simplistic. A number of researchers have argued over the past few years that during the Optional Infinitives/OI stage of Wexler (1994) children may ‘have’ UNDERSPECIFIED functional categories. For example, Schütze & Wexler (1996) and Schütze (1997) argue that the category INFL at the OI stage may be underspecified in respect of its tense/agreement features: so, for example, in He cries, INFL is specified for both tense and agreement (with agreement triggering nominative casemarking of the subject he); in He cry (e.g. in reply to a question like ‘What did the baby do?’) INFL is specified for agreement but not tense; and in Him cried, INFL is specified for tense but not agreement. Given Chomsky's (1995) view that categories are sets of features, it follows that an underspecified child INFL constituent cannot in principle represent THE SAME CONSTITUENT as an adult INFL fully specified for tense and agreement (and perhaps mood/finiteness as well), but rather is a proto-INFL. This in turn suggests that children have to learn to build up feature complexes associated with functional categories ‘one feature at a time’ – and that the OI stage represents a period when certain features of INFL are taken to be optional. In other words, it may be that Peters' observation that ‘children must construct their grammatical categories on the basis of gradual learning’ is as true of functional categories as it is of substantive categories. On this view, functional categories MATURE. Indeed, this conclusion seems to be a matter of virtual conceptual necessity if we follow Chomsky (1999) in positing that learners have to ASSEMBLE lexical items from sets of features provided by the Language Faculty, and if we assume that feature-assembly is not instantaneous.