Such a way of describing the situation should not be too surprising to those familiar with the history of psychology at Berkeley, where, after all, Edward Tolman has given his name to the psychology building; but I want to stress that the hypotheses I am discussing are qualitatively different in complexity and intricacy from anything that was considered in the classical discussions of learning. As I have now emphasised several times, there seems to be little useful analogy between the theory of grammar that a person has internalised and that provides the basis for his normal, creative use of language, and any other cognitive system that has so far been isolated and described; Similarly, there is little useful analogy between the schema of universal grammar that we must, I believe, assign to the mind as an innate character, and any other known system of mental organisation. It is quite possible that the lack of analogy testifies to our ignorance of other aspects of mental function, rather than to the absolute uniqueness of linguistic structure; but the fact is that we have, for the moment, no objective reason for supposing this to be true.
The way in which I have been describing acquisition of knowledge of language calls to mind a very interesting and rather neglected lecture given by Charles Sanders Peirce more than fifty years ago, in which he developed some rather similar notions about acquisition of knowledge in general. Peirce argued that the general limits of human intelligence are much more narrow than might be suggested by romantic assumptions about the limitless perfectibility of man (or, for that matter, than are suggested by his own "pragmaticist" conceptions of the course of scientific progress in his better-known philosophical studies). He held that innate limitations on admissible hypotheses are a precondition for successful theory construction, and that the "guessing instinct" that provides hypotheses makes use of inductive procedures only for "corrective action," Peirce maintained in this lecture that the history of early science shows that something approximating a correct theory was discovered with remarkable ease and rapidity, on the basis of highly inadequate data, as soon as certain problems were faced; he noted "how few were the guesses that men of surpassing genius had to make before they rightly guessed the laws of nature." And, he asked, "How was it that man was ever led to entertain that true theory? You cannot say that it happened by chance, because the chances are too overwhelmingly against the single true theory in the twenty or thirty thousand years during which man has been a thinking animal, ever having come into any man's head." A fortiori, the chances are even more overwhelmingly against the true theory of each language ever having come into the head of every four-year-old child. Continuing with Peirce: "Man's mind has a natural adaptation to imagining correct theories of some kinds.... If man had not the gift of a mind adapted to his requirements, he could not have acquired any knowledge." Correspondingly, in our present case, it seems that knowledge of a languages grammar — can be acquired only by an organism that is "preset" with a severe restriction on the form of grammar. This innate restriction is a precondition, in the Kantian sense, for linguistic experience, and it appears to be the critical factor in determining the course and result of language learning. The child cannot know at birth which language he is to learn, but he must know that its grammar must be of a predetermined form that excludes many imaginable languages. Having selected a permissible hypothesis, he can use inductive evidence for corrective action, confirming or disconfirming his choice. Once the hypothesis is sufficiently well confirmed, the child knows the language defined by this hypothesis; consequently, his knowledge extends enormously beyond his experience and, in fact, leads him to characterise much of the data of experience as defective and deviant.
Peirce regarded inductive processes as rather marginal to the acquisition of knowledge; in his words, "Induction has no originality in it, but only tests a suggestion already made." To understand how knowledge is acquired, in the rationalist view that Peirce outlined, we must penetrate the mysteries of what he called "abduction," and we must discover that which "gives a rule to abduction and so puts a limit upon admissible hypotheses." Peirce maintained that the search for principles of abduction leads us to the study of innate ideas, which provide the instinctive structure of human intelligence. But Peirce was no dualist in the Cartesian sense; he argued (not very persuasively, in my opinion) that there is a significant analogy between human intelligence, with its abductive restrictions, and animal instinct. Thus, he maintained that man discovered certain true theories only because his "instincts must have involved from the beginning certain tendencies to think truly" about certain specific matters; similarly, "You cannot seriously think that every little chicken that is hatched, has to rummage through all possible theories until it lights upon the good idea of picking up something and eating it. On the contrary, you think that the chicken has an innate idea of doing this; that is to say, that it can think of this, but has no faculty of thinking anything else.... But if you are going to think every poor chicken endowed with an innate tendency towards a positive truth, why should you think to man alone this gift is denied?"
No one took up Peirce's challenge to develop a theory of abduction, to determine those principles that limit the admissible hypotheses or present them in a certain order. Even today, this remains a task for the future. It is a task that need not be undertaken if empiricist psychological doctrine can be substantiated; therefore, it is of great importance to subject this doctrine to rational analysis, as has been done, in part, in the study of language. I would like to repeat that it was the great merit of structural linguistics, as of Hullian learning theory in its early stages and of several other modern developments, to have given precise form to certain empiricist assumptions." Where this step has been taken, the inadequacy of the postulated mechanisms has been clearly demonstrated, and, in the case of language at least, we can even begin to see just why any methods of this sort must fail — for example, because they cannot, in principle, provide for the properties of deep structures and the abstract operations of formal grammar. Speculating about the future, I think it is not unlikely that the dogmatic character of the general empiricist framework and its inadequacy to human and animal intelligence will gradually become more evident as specific realisations, such as taxonomic linguistics, behaviourist learning theory, and the perception models," heuristic methods, and "general problem solvers" of the early enthusiasts of "artificial intelligence," are successively rejected on empirical grounds when they are made precise and on grounds of vacuity when they are left vague.