Noam Chomsky, the linguist and social critic, turned 80 in December. Few who read him are indifferent to his message. A biographer, Robert Barsky, even declares: "Chomsky is one of this [past] century's most important figures, and has been described as one who will be for future generations what Galileo, Descartes, Newton, Mozart or Picasso have been for ours."

Leave aside the hyperbole and consider the improbability of its recipient. Chomsky has been the most influential figure in theoretical linguistics since the 1960s. His idea that human languages are the realisation of an innate language faculty is part of our intellectual culture. But this is a specialised discipline, in which Chomsky's ideas are far from universally held. Scholars once close to him, such as the cognitive scientist Steven Pinker, have diverged from important elements of his ideas.

Chomsky's popularity derives primarily from his political output: a stream of books, essays and interviews condemning America's role in the international order and its supposed marginalising of dissent at home. Chomsky sees the US as "a leading terrorist state", insisting he derives this from consistent application of a universal standard.

This is the background to two persistent myths about Chomsky. The first is that his political views are distinct from, and even aberrant compared with, his seminal work in linguistics. An example of this position is Richard Posner, who in his book Public Intellectuals notes that Chomsky's political writing "has taken a great deal of time away from his immensely distinguished academic career, and yet has received little public attention, much of it derisory".

The second is that, while Chomsky's political vision may be flawed in its absolutism, it nonetheless possesses an appealing moral consistency. In the New York Times, Obama adviser Samantha Power urged: "It is essential to demand, as Chomsky does, that a country with the might of the United States stop being so selective in applying its principles."

Chomsky's political output is consistent with the rest of his oeuvre in one important respect: the method of argumentation. Across disciplines, he has long employed a variety of unscholarly techniques to insulate his conclusions from criticism. The linguist George Lakoff once identified Chomsky's tendency to "fight dirty when he argues. He uses every trick in the book." In the current issue of the journal Artificial Intelligence, Margaret Boden, Professor of Cognitive Science at Sussex University, notesthat a review of her book by Chomsky is "a sadly unscholarly piece, guaranteed to mislead its readers about both the tone and the content of the book. It is also defamatory." In politics, Chomsky's preferred technique is vituperative abuse of his opponents. Take a few examples. Thomas Friedman of the New York Times is an "astonishing racist and megalomaniac". In disputing Chomsky's analogy between 9/11 and President Clinton's attack on a pharmaceutical factory in Sudan, Christopher Hitchens "must be unaware that he is expressing such racist contempt". The French nation collectively has a "highly parochial and remarkably illiterate culture".

The irony of Chomskyan invective in the political sphere is that it is highly selective. Chomsky has never regretted his intervention in the 1980s on behalf of a Holocaust denier, Robert Faurisson. Chomsky has no sympathy with Holocaust denial, and if he had stuck to defending Faurisson's right to free expression he would have been right and principled. Instead, he wrote: "As far as I can determine, [Faurisson] is a relatively apolitical liberal of some sort." The point here is not the perversity of the judgement. It is the way in which Chomsky espouses supposedly universal principles while extravagantly failing to apply them. Liberals and left-wingers who see value in US interventionism are racists, frauds, apologists for state terror and so on. Yet a man like Faurisson who exemplifies all of these qualities is regarded differently.

Consider, too, Chomsky's writings on Indochina, the issue on which he became famous as a political controversialist. He did not only excoriate an unjust and brutal US war. He derided refugee accounts of horrors after the fall of Cambodia, pointedly referring to "alleged Khmer Rouge atrocities" and disputing a comparison of Pol Pot's rule to Nazi Germany. In an interview last year, Chomsky characteristically congratulated himself on the astuteness of this analysis, declaring: "If we were to rewrite it now, we'd do it exactly the same way."

Chomsky's output is vast, and not always wrong. He was early and right in condemning Western acquiescence in Indonesia's subjugation of East Timor (though typically he cannot now acknowledge that the reversal of that policy has provided a casus belli for Islamist terrorism). But he cannot be accused of disinterested opposition to oppression. His political writings will last, if at all, only as a monument to Xenophon's definition of the sophist as one who sells wisdom to pupils for pay.