7.3.4 The interaction between wh-movement and inversion
This is a puzzle for which I have no real account. Apparently it is a very general condition that a CP can contain either an overt operator or an overt complementiser. We will see that this extends to other clauses too. Moreover, it appears not to be violated by an auxiliary moving to C. The condition has been known as the Doubly Filled COMP Filter since (1977) when it was introduced by Chomsky and Lasnik. However, this stipulatory account has never been superseded by anything more explanatory. I will therefore adopt the Doubly Filled COMP Filter as a condition on the well-formedness of structures in lieu of a proper explanation:
One possible solution to both these problems would be to claim that PRO doesn’t avoid Case positions per se, but has to sit in special Case positions which up to now have been assumed not to be Case-marked, but in fact might be assigned a special Case, applicable only for PRO. Chomsky and Lasnik (1989) proposed that PRO must sit in special Case marked positions. They argue that the subjects of certain non-finite clauses are not Caseless but that what they term ‘Null Case’ is assigned to them. Only PRO can bear Null Case and Null Case is the only Case that PRO can bear. Thus PRO will not be able to sit where overt DPs go as these will be Case marked with something other than Null Case. Moreover no overt DP can sit in a position in which it would be assigned Null Case as this is not ‘strong’ enough to satisfy the Case Filter. The good thing about this assumption is that it predicts complementary distribution between overt DPs and PRO but does not force us to assume that PRO can occupy any position in which we cannot find an overt DP. From this perspective, then, PRO cannot sit in a position to which no Case is assigned, as in (73).