[…] we can say that the child has developed and internally represented a generative grammar [...] He has done this on the basis of observation of what we may call 'primary linguistic data'. This must include examples of linguistic performance that are taken to be well-formed sentences, and may include also examples designated as non-sentences, and no doubt much other information of the sort that is required for language learning, whatever this may be.
- Chomsky (1969), Pag. 25

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 [...] 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.
- Chomsky (1969), Pag. 31-32