No two children, even those learning what is regarded as the same language, have exactly the same experiences of language. Most children have entirely different experiences. And yet they all converge on the same grammar. The same competence is shared. This is known as ‘the logical problem of language acquisition’. Competent speakers can agree or disagree about plays on words and other sorts of jokes and the exact meaning of business contracts and political resolutions and agreements. And if they don’t agree, they can, and often do, argue about the meanings.

People brought up in an English-speaking environment mostly end up agreeing that:

• There is no difference in meaning between “You believe that it’s true?” and “You believe it’s true?”
• “The rabbit is ready to eat” means two things, one good for the rabbit, one not.
• “Mightn’t the ball that won the match that the bookie keeps talking about have been being examined by the umpire?” is meaningful (even if it clunks more than somewhat).

For any language, agreement on such points is possible in principle. This is shown most dramatically in the language development of two normally-developing brothers, Joe and Arthur, two and half years apart in age, analysed more fully and in more detail in the proposal here,. At around two years and ten months,  Joe said “I want to stand on the chair to see what’s happening”. At the same age,  Frank said “I want to sit where Joe’s been sitting.” In both children, these were thes first sentences with multiple embeddings, in both cases with the most deeply embedded clause introduced by an Wh word, what in Joe’s case, and where in Frank’s. In both cases, the structure is completely grammatical with a contracted auxiliary ‘s forming a tensed structure.

Obviously, what Joe was saying was an important part of Frank’s language experience. But while Frank may have taken on board many of Joe’s interests, it does not seem plausible that this extended to such trivial curiosities as the formal structures of their grammar at the same chronological age.

The exactness of the similarity between these structures (only noticed forty years later when the data came to be ansalysed), and their complete grammaticality at a point when the first embeddings were just first appearing, does strongly suggest that there is some significance in them. By the proposal here, these structures signal a critical point in the mastery of ‘Universal Grammar’.

By the same token, this is only possible if the essential structures, are well defined, and not some random chunks of language experience, as proposed by Mike Tomasello (2003), Ben Ambridge (2004), and others.

The end-state