  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  they'd never succeed in learning a language. The other would be if  there were something that ensured that when they generalize  beyond the data, they don't overgeneralize, something, that  is, that ensures that children don't make errors that they could only correct on the basis of negative evidence. According to the  linguistic nativist, this something is innate knowledge of UG.