  The argument from (1), (2), and (3) to (4) appears valid. However, as is implicit in my discussion of premiss (2), an equivocation between  different senses of ‘learning’ threatens. What (1)-(3)  show, if true, is that grammar G can't be learned from the  pld by a learner using a ‘Popperian’ learning strategy, that is, a strategy of ‘bold conjecture’ and refutation. What (4) concludes, however, is that G is unlearnable, period, from the pld — a move that several authors, particularly connectionists, have objected to. (See especially Elman et al. 1996 and Elman 1998 for criticisms of Chomskyan nativism along these lines; see Marcus 1998 and 2001 for responses.)