In sum: a child who is in situation (iv) — a child whose grammar ‘overgenerates’ — would need negative evidence in order to recover from her error. Negative evidence, however, does not appear to exist. Since children do manage to learn languages, they must never get themselves into situation (iv): they must never need to ‘unlearn’ any grammatical rules. There are two ways they could do this. One would be never to generalize beyond the data at all. But clearly, children do generalize, else