Colorless green ideas sleep furiously.

Noam Chomsky coined this sentence in the mid-1950s while developing generative grammar and arguing against behaviorist and purely statistical accounts of language. He used it as an example of a string that is syntactically well-formed in English (it follows familiar grammatical patterns) yet semantically anomalous, illustrating that grammaticality cannot be reduced to meaningfulness or to observed word co-occurrence frequencies. The example became widely cited in linguistics and cognitive science as a compact demonstration that speakers’ knowledge of grammar includes abstract structural rules beyond surface habits or probabilities.

The sentence is designed to separate form from meaning. Each word is ordinary, and the overall structure—adjective + adjective + noun + verb + adverb—sounds like a legitimate English clause, but the combined concepts clash (“colorless” vs. “green,” “ideas” that “sleep,” and doing so “furiously”). Chomsky’s point is that native speakers can recognize grammatical well-formedness even when a sentence is nonsensical, implying that linguistic competence involves internalized rules and hierarchical structure. The line also became a touchstone in debates about whether language understanding is primarily rule-governed, usage-based, or statistical.

“Furiously sleep ideas green colorless.” (used as an ungrammatical contrast in the same discussion)