As I alluded to before, Grace and Virginia Kennedy did not of course create their private language out of whole cloth. Feral children, those rare "wild children" who spend their earliest years separated from all human contact, do not use language. They behave pretty much as nonhuman animals. Their sad example underscores the fact that human beings are, as Aristotle said, social animals. This does not mean merely that we like to hang out together and chat; to be a social animal is something far more profound than that.
To be a social animal is to be a creature whose very nature depends on its sociality. In other words, a human being in complete and utter isolation is hardly even a human being. Perhaps more accurately, he is a human being who is greatly crippled, whose potential as a human being is not made actual, whose horizons and powers are severely limited and blocked. A human being living in complete isolation is not truly free, because he is not able to use his full human powers nor realize his full potential as a human being. Those accomplishments require society and culture, which by definition entail relationship with other human beings.
The Kennedy girls, despite their relative social isolation, were not of course feral children. They existed on the margins of human society, but still had some contact with it. In this way they were exposed to the phenomenon of language itself, and to at least snatches of the specific languages of English and German.
Language is one of the defining features of society. Even when people are physically proximate, if they do not communicate with each other they are hardly being social. It is interesting, and not coincidental, that the word "commune" is at the heart of the word "communication", as it is in the words "community" and "communion". Communication, or language in its broadest sense, is the glue that binds us together and that makes a society out of individual human beings; and the ideas and information expressed by that language constitute that society's culture, its ability to think and act above the level of mere natural instinct and mindless bodily impulse. It is what makes us civilized; it is, to a large degree, what makes us human.
We will never know the exact process by which Grace and Virginia created their peculiar form of speech. It would of course be entirely out of the question morally to subject children to such an "experiment" so that we might observe how such a process of language acquisition under those conditions unfolds, so we must go on inference and speculation. No doubt the girls heard their parents and grandmother speak; perhaps they heard the radio or watched television; in any case, we can only assume that they combined imitation of what they heard with their own imagination.
Imitation and imagination are both entirely natural and wondrously powerful capabilities of children, and both are essential to learning. One thing that seems clear to me is that, for the Kennedy girls, imagination played a far more prominent role in their language acquisition process, relative to imitation, than it does for most children. To me, this is the truly fascinating aspect of their feat: whereas for most children, the acquisition of vocabulary, pronunciation, and grammar is by way of imitation and an intuitive understanding of that which is imitated, Virginia and Grace had to rely to a very large extent on their own native imagination and creativity.
The girls were forced to rely upon the resources of their own minds, those minds having been shaped only vaguely by the larger human world. Yet the fact that they produced a complex language containing unique vocabulary and grammatical features illustrates the marvelous capacity for language that is inherent in the human mind. Like imitation, imagination too must rely upon experience, but it transforms that experience into something new by way of a mysterious and deeply intuitive process of creation. Whereas most children rely primarily on imitation to learn language, which ensures that the mother tongue is faithfully reproduced in the next generation, the Kennedy girls, having relatively little linguistic experience to imitate, had to fill in the gaps with their own fancy.
They surely did all of this in a largely unselfconscious way, almost as a form of play. "What is that?" my two-year-old daughter often asks me, wanting to know the word for something. With no adults around to tell them, Grace and Virginia had to make something up. Perhaps it was a misremembered word that they had heard a grown-up say, or that they had heard on TV. Or perhaps it was purely made up, a gibberish sound that became a meaningful word, because they assigned that sound a particular meaning.
In this way, the Kennedy girls' language became a real language. It was a mode of communication shared between two human beings, only "private" in a relative sense, but still a social and collaborative construction and tool for sharing ideas, thoughts, and feelings. Virginia and Grace understood each other. No one else did, but a language is made no less valid just because you cannot understand it or speak it. These words and sentences were meaningful and perfectly intelligible to Grace and Virginia Kennedy--or, as they chose to name and identify themselves, Poto and Cabengo.