Why is the phrase “colourless green ideas sleep furiously” considered meaningless even though it follows grammatical rules?

The phrase “colourless green ideas sleep furiously” is considered meaningless—or anomalous—because it lacks semantic coherence despite being syntactically well-formed.1

In the study of semantics, a distinction is made between the rules that govern the structure of a sentence (syntax) and the rules that govern the assignment of meaning (semantics). While the sentence follows the grammatical rules of English—placing adjectives, nouns, verbs, and adverbs in their appropriate positions—the individual words are semantically incompatible.231

For example, the adjectives "colourless" and "green" are contradictory, and the concept of "ideas" cannot logically "sleep," nor can such an action be performed "furiously." Because these terms cannot be reconciled to form a coherent thought or represent a possible state of affairs, the sentence fails to convey a meaningful proposition. This example highlights that a sentence can be perfectly grammatical while remaining entirely devoid of sense, demonstrating that syntax and semantics are distinct components of language.12