  For consider: The fact (i) that the pld are finite whereas  natural languages are infinite shows that children must be  generalizing beyond the data when they are learning their language's  grammar: they must be proposing rules that cover as-yet unheard  utterances. This, however, opens up room for error. In order to  recover from particular sorts of error, children would need access to particular kinds of data. If those data don't exist, as (ii) asserts, then children would not be able to correct their mistakes. Thus,  since children do eventually converge on the correct grammar for  their language, they mustn't be making those sorts of errors in the  first place: something must be stopping them from making  generalizations that they cannot correct on the basis of the  pld.