[TimMason]

how do you account for the fact that I learned French as a child, and you learned English (presumably)? Neither of us invented a new language.

What some linguists believe happens is that children re-invent the language. Up until the age of about 8 or 9, they are able to take even rather poor language input and turn it into a flexible, working language. That's how you get from a pidgin to a creole. One interesting example is how deaf children in Nicaragua were able to create a sign language once they were brought together. The government decided to round up all the deaf children, and put them in a specialized school where they would learn Spanish. They didn't learn Spanish very well, but the younger ones took the rather basic signs that the adolescents had worked out for themselves, and turned them into a language. You can read about this in Pinker's 'The Language Instinct' which, once again, I do highly recommend. (There is a French edition, published by Odile Jacob).

Children don't directly imitate their parents when they speak. They have their own grammars (often, parents will correct this, but the children resist correction). Gradually, their grammar comes to resemble that of the parents, but they get there by their own route. That's why Chomsky and Pinker argue that the grammar is innate (although this is a simplification of their position).

If anything like this is actually the case, then it has philosophical ramifications. It suggests that human beings are far more autonomous than earlier models of learning would have it. But at the same time, it also suggests that the child needs to be placed within a sufficiently rich network of social relations with both adults and other children (for bilingual children like mine, it is the language of the school that usually becomes the dominant tongue, rather than the language of one or the other of the parents).