As in the cases of the cell and of embryonic development, therefore, we need a new theoretical framework for mental development, even if this means that we have to abandon the nature-nurture scheme that has been imposed for centuries on our approach to the human mind.

The semantic theory of mental development

It has been held for centuries that mind and body are divided by an unbridgeable gulf, but in reality there is no actual proof that they develop with totally different mechanisms. There are, on the contrary, some intriguing common features in their developments.
We have seen that a “universal grammar” must appear in a very early phase of mind development, and in that phase we can rightly say that a child has a species-specific mind, or a specietypic mind, because that mental state is shared by all members of our species.
This suggests immediately that the “specietypic stage” of mental development is comparable with the “phylotypic stage” of embryonic development. In both cases, it is necessary that all members of a taxonomic group pass through a common phase of development before they begin developing individual characteristics. Even mental development, in other words, is a sequence of two processes: one that builds the specietypic mind, and the other that goes on from that stage and builds the individual mind.
We have seen, furthermore, that the phylotypic stage of embryonic development is very short, but the body plan that is built in that brief interval remains for life, and acts as an organizing center for the individual body. And the same is true for the mind: the phase of the specietypic mind is transient, but the universal grammar that is built in that brief time does not disappear, and becomes the organizing center of the individual mind.
We have also seen that, in the language learning period, a child actually encounters only an extremely limited and erratic sample of words and phrases, and yet, in the end, all children in a population learn the same language, and spontaneously invent countless rules that nobody thaught them.