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."