As linguists prepare for their annual conference, Neil Smith and Ian Robinson do battle over the verbal high ground, debating the legacy of a linguistic icon

Noam Chomsky has shown that there is really only one human language: that the immense complexity of the innumerable languages we hear around us must be variations on a single theme. He has revolutionised linguistics and in so doing has set a cat among the philosophical pigeons.

He has resurrected the theory of innate ideas, demonstrating that a substantial part of our knowledge is genetically determined; he has reinstated rationalist ideas that go back centuries but that had fallen into disrepute; and he has provided evidence that "unconscious knowledge"

is what underlies our ability to speak and understand. He has overturned the dominant school of behaviourism in psychology and has returned the mind to its position of pre-eminence in the study of humankind. In short, Chomsky has changed the way we think of ourselves, gaining a position in the history of ideas on a par with that of Darwin or Descartes. And he has done this while devoting most of his time to dissident politics and activism.

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The Chomskyan era is traditionally dated from the publication of Syntactic Structures , which celebrates its 50th anniversary next year, but this is misleading. The seeds of Chomsky's "revolution" are already visible in his Morphophonemics of Modern Hebrew , started when he was an undergraduate and refined as a masters dissertation in 1951. Many of the technical innovations associated with the generative enterprise appear in this early work, but many of the conceptual innovations that extend its interest to philosophy and psychology do not appear until his review of B.

F. Skinner's Verbal Behaviour in 1959. These philosophical developments, which sounded the death knell of behaviourism, were consolidated and spelt out in 1965 in Aspects of the Theory of Syntax , the book that defined the "standard theory" of generative grammar.

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The distinction between the technical and the conceptual is important, even if the boundary is sometimes fuzzy. As a graduate student doing fieldwork on the little-known Nigerian language Nupe, I was repeatedly confronted by problems where pre-Chomskyan linguistics was simply incapable of providing tools adequate to the descriptive task in hand. Consider "unbounded movement", which functions much the same way in Nupe and English. In a question such as "Which politician did the judge suggest the witness had bribed?", the "politician" is the object of "bribed", just as in the corresponding statement "The judge suggested the witness had bribed the politician." Such examples were formally unanalysable and a major contribution of Syntactic Structures was that it provided the technical basis, transformations, for solving such descriptive problems. This technical advance, produced by building on results in mathematics and logic, could then be exploited by all linguists, whether or not they were persuaded by, or even interested in, the philosophical and psychological implications of Chomsky's work.

For Chomsky, language is a mirror of the mind, and the study of language constitutes a sub-part of psychology, rather than of mathematics or literary studies. Ultimately, it is an aspect of human biology, to be studied by the normal canons of the natural (cognitive) sciences. What this means is that linguistics is assimilated to the Galilean tradition and is committed to devising explanatory theories and making testable predictions.

Just as Galileo is considered the father of the experimental method, which underlies all of modern science, so Chomsky is considered the father of the cognitive revolution, which is extending that methodology to the study of mind. One implication of this development is that the study of language may have nothing to say about areas that are of deep concern to other linguists. Chomsky has had little to say about language in education or pathology or literature. Some generative linguists do contribute in such areas but, in general, this is not dependent on Chomsky's core ideas. His generative grammar provides a theory of one human faculty, the module of the mind devoted to linguistic competence, which can be unified with, though not reduced to, a variety of other theories: of linguistic performance; of theory of mind; of memory.

Chomsky has always been preoccupied with explanation rather than description, and his technical innovations have always been motivated by the attempt to illuminate two puzzles: how adults can use language creatively, and how it is possible for children to acquire their first language. The term "creative" means merely that speakers are able to utter and understand any of an infinite set of possible sentences, many of which are created anew on the appropriate occasion. Because the set is infinite and the utterances may contain recognisable mistakes, they could not simply have been memorised but must be the result of a set of mentally represented rules. Such rules appear to be at once surprisingly complex and surprisingly similar from language to language. The complexity of this knowledge, uncovered in remarkable detail in 50 years of intensive work, gives rise to the problem of how we can acquire it in the few years at our disposal. The unexceptionable response is that our language faculty must result from the interplay of the innate and the learned. What takes the claim beyond banality is the detailed proposal for what precisely is innate.

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Consider "Plato's problem": how can we know so much when the evidence is so meagre; in particular, how can we acquire our first language so effortlessly? Under experimental conditions, children of two or three can successfully identify the correct picture corresponding to sentences such as "Is Goldilocks touching herself?", as opposed to "Is Goldilocks touching her?". They seem to have mastered the syntax of reflexives at a stage when doing up their buttons is beyond them. The explanation comes from the framework known as "principles and parameters": we are born with a mind constrained by universal principles, characteristic of all languages and with tacit knowledge of the possible ways languages can differ from each other. These differences, or "parameters", are like switches that the child can set one way or the other on the basis of simple evidence in the language it hears around it. Moreover, given the differences between us and chimpanzees, the language faculty, with its apparently awesome complexity, must have evolved fairly recently. Corroboration of this position comes from recent work in genetics, where we are now able to quantify the respective contributions of nature and nurture for particular abilities.

Chomsky's arguments for the partial innateness of the language faculty are controversial and have dominated philosophical discussion of his work. Some of the controversy is resolved in The Minimalist Program (1995), which suggests that what is both innate and strictly linguistic is extremely limited, and is imposed by the need for "legibility". The products of the language faculty, representations of the sounds and meanings of sentences, derive many of their properties from the interfaces with which they have to interact: systems of perception and production on the one hand; systems of thought and memory on the other. A crucial goal is then to determine what is unique to language and unique to humans. Both strands are important: there has been much recent discussion of the FOXP2 gene, disruption to which causes serious linguistic problems. But we share this gene with mice, so it is not unique to humans. Similarly, the infinite expressive power of language is something shared with mathematics and music, so "infinity" cannot be unique to the human language faculty. What is peculiar to human language may, perhaps, be limited to recursion: the ability to embed one sentence in another. But this property plausibly derives from whatever system we use to think with - the language of thought. If so, the only thing unique to human language would be the form of the mapping that links linguistic expressions to the interfaces. Chomsky has suggested that language is "perfect": the mapping is the simplest that is logically possible given the nature of the requirement of legibility. Determining what is innate is still an important preoccupation, but the status of what is innate, linguistic or more generally cognitive, changes as we extend the investigation of the human mind.

Many of Chomsky's ideas have remained unchanged, and Cartesian, for half a century: an emphasis on universals, on rationality, on modularity, on the central importance of psychological reality and of philosophical realism. But there have been constant surprises along the way, with radically new suggestions delighting or outraging the academic community. Let me finish by mentioning two aspects of Chomsky's legacy. First, it would be remiss totally to neglect the other half of his work - his activism. He has, after all, published more books on politics than linguistics. Chomsky has always said that his radical politics are independent of his linguistics, but there are unifying threads of modularity, creativity and rationality, which underpin both strands of his work.

Second, it is relevant to stress his role in fomenting the diversity of modern linguistics. The field is vibrant, there are countless approaches, rivalries, theories and anti-theories. The monolith of the Sixties (if it ever existed) has crumbled. Many linguists owe their preoccupations to Chomsky's work, even when they distance themselves from him. Others owe little to his work except as a focus for their opposition - bemused, contemptuous or indifferent. Even here the visibility of linguistics and its centrality to the study of cognition are part of Chomsky's legacy.

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Neil Smith is a professor in the department of phonetics and linguistics at University College London, author of Chomsky: Ideas and Ideals (Cambridge University Press, ?17.99) and has contributed a chapter to the recently published Encyclopedia of Language and Linguistics (Elsevier).