The Pirahã are a small indigenous community of around 700 people living along the Maici River, a tributary of the Amazon, in the western Brazilian state of Amazonas. Their language is among the most intensively studied in the world. Not for its number of speakers, but for what it appears to lack.

Pirahã has no number words. The two terms previously translated as "one" and "two" are now understood to mean roughly "smaller amount" and "larger amount." In experimental tasks, speakers cannot reliably perform exact counting above three or four. The language has no fixed color vocabulary. Speakers describe colors by comparison — like blood, like the sky. There is no straightforward way to express past tense; time is anchored to events the speaker has directly witnessed. There are no traditional creation myths, no fiction, and no narratives that go beyond living memory.

The American linguist Daniel Everett went to live among the Pirahã in 1977, originally as a Christian missionary with the Summer Institute of Linguistics. He spent decades there, eventually leaving the church but continuing the research. His most controversial claim is that Pirahã grammar does not allow recursion — the embedding of phrases inside other phrases, as in English "the dog that the cat chased ran away."

The claim matters because the linguist Noam Chomsky has argued for half a century that recursion is the one defining feature of all human language, an innate property of the human mind. Pirahã, Everett contends, is the counterexample.

The dispute is unresolved. Other linguists have offered alternative analyses of the same data, and fieldwork by other researchers has produced conflicting interpretations. For now, a small Amazonian community of a few hundred speakers sits at the center of one of the longest-running debates in modern linguistics.