 Linguists have given considerable attention to considerations of in-principle learnability—not so much the course of language acquisition as tracked empirically (the work of developmental psycholinguists) but the question of how languages of the human sort could possibly be learned by any kind of learner. The topic was placed squarely on the agenda by Chomsky (1965); and a hugely influential mathematical linguistics paper by Gold (1967)has dominated much of the subsequent discussion.