  We have already noted one facet of this exceptionalism: the fact that grammars are very complex. But there are also unexpected singularities in how children learn; in the learning process itself. Each child is exposed to an idiosyncratic sample of the language (their primary linguistic data). Each sample is compatible with any number of non-equivalent grammars that all generate the pld sample so far, but give different verdicts about new cases not in the pld. We might therefore expect (i) that the grammar a child acquires reflects the idiosyncrasies of the pld the child was exposed to, (ii) that, as a consequence, children will disagree about what is and what is not grammatical, and (iii) that adults will therefore have to correct them to smooth out errors that reflect those idiosyncrasies. But this, Chomsky argued, is not what we find (Chomsky 1965). Children learning a language somehow converge on the same grammar, as evidenced by their agreement about well-formedness, and by the distinctive types of errors they make and don’t make in the course of learning. If this is right, it suggests that the child must have prior information that somehow constrains or orders the hypothesis space that steers the child to the right grammar, and it is hard to see how this information can be acquired through experience. Furthermore, the pld contain ungrammatical and incomplete sentences, but children somehow filter out this noise, and do so without explicit instructions or feedback. There are a number of other striking features about language learning that Chomsky drew attention to: (1) it is acquired rapidly, (2) the speed of acquisition does not correlate with intelligence, (3) it does not require reinforcement or extensive explicit training, and (4) it is acquired in a critical period—a relatively fixed window in the maturation process—during which other less complex systems (counting, for instance—see below section 2.3) cannot be mastered. Each of these claims has prompted a long trail of experimentation and theory construction, and all remain controversial (see, for example, the discussion in Menn et al. 2003). But their overall effect was to single out language learning as exceptional, and perhaps unique. Chomsky himself marked this difference by speaking of language acquisition and contrasting it with learning, a term he reserved for induction-based processes.