Piaget concluded that babies aren\'t just born in possession of adult knowledge, either from a past life or from DNA. Instead, Piaget thought that children must have powerful learning mechanisms that allow them to construct new pictures of the world, pictures that might be very different from the adult picture. When we learn about the world, when we do science. for example, we don\'t just hit the right answer once and for all. Rather, there is a very gradual unfolding sequence of corrected errors, expanded ideas, and revised misconceptions as we approach more and more nearly to the truth. That was what the Piagets saw as they watched their babies make their way through infancy.

But Piaget also thought that learning was just as rooted in biology as any innate idea in the genetic code. He often used the metaphor of digestion: babies\' minds assimilated information the way babies\' bodies assimilated milk. For Piaget, learning was as natural as eating. This idea is the second element in the new developmental science.

Learning is natural, innate.

Consider first the nature of primary linguistic data. This consists of a finite amount of information about sentences, which, furthermore, must be rather restricted in scope, considering the time limitations that are in effect, and fairly degenerate in quality (cf. note 1 4). For example, certain signals might be accepted as properly formed sentences, while others are classed as nonsentences, as a result of correction of the learner's attempts on the part of the linguistic community. Furthermore, the conditions of use might be such as to require that structural descriptions be assigned to these objects in certain ways. That the latter is a prerequisite for language acquisition seems to follow from the widely accepted (but. for the moment. quite unsupported) view that there must be a partially semantic basis for the acquisition of syntax or for the justification of hypotheses about the syntactic component of a grammar. Incidentally. it is often not realized how strong a claim this is about the innate concept-forming abilities of the child and the system of linguistic universals that these abilities imply. Thus what is maintained. presumably. is that the child has an innate theory of potential structural descriptions that is sufficiently rich and fully developed so that he is able to determine. from a real situation in which a signal occurs, which structural descriptions may be appropriate to this signal, and also that he is able to do this in part in advance of any assumption as to the linguistic structure of this signal. To say that the assumption about innate capacity is extremely strong is, of course. not to say that it is incorrect. Let us, in any event, assume tentatively that the primary linguistic data consist of signals classified as sentences and nonsentences. and a partial and tentative pairing of signals with structural descriptions.

A language-acquisition device that meets conditions (i)-(iv) is capable of utilizing such primary linguistic data as the empirical basis for language learning. This device must search through the set of possible hypotheses GI• G2• • • • • which are available to it by virtue of condition (Hi), and must select grammars that are compatible with the primary linguistic data, represented in terms of (i) and (ii). It is possible to test compatibility by virtue of the fact that the device meets condition (iv). The device would then select one of these potential grammars by the evaluation measure guaranteed by (V).19 The selected grammar now provides the device with a method for interpreting an arbitrary sentence, by virtue of (H) and (iv) . That is to say, the device has now constructed a theory of the language of which the primary linguistic data are a sample. The theory that the device has now selected and internally represented specifies its tacit competence, its knowledge of the language. The child who acquires a language in this way of course knows a great deal more than he has "learned." His knowledge of the language, as this is determined by his internalized grammar, goes far beyond the presented primary linguistic data and is in no sense an " inductive generalization" from these data.

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To acquire language, a child must devise a hypothesis compatible with presented data - he must select from the store of potential grammars a specific one that is appropriate to the data available to him. It is logically possible that the data might be sufficiently rich and the class of potential grammars sufficiently limited so that no more than a single permitted grammar will be compatible with the available data at the moment of successful language acquisition, in our idealized "instantaneous" model (cf. notes 1 9 and 2 2) . In this case, no evaluation procedure will be necessary as a part of linguistic theory - that is, as an innate property of an organism or a device capable of language acquisition. It is rather difficult to imagine how in detail this logical possibility might be realized, and all concrete attempts to formulate an empirically adequate linguistic theory certainly leave ample room for mutually inconsistent grammars, all compatible with primary data of any conceivable sort. All such theories therefore require supplementation by an evaluation measure if language acquisition is to be accounted for and selection of specific grammars is to be justified; and I shall continue to assume tentatively, as heretofore, that this is an empirical fact about the innate human facu lte de langage and consequently about general linguistic theory as well.

It is possible that children are born with the ability to acquire language.