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Grelling–Nelson paradox, also known as the Grelling–Nelson paradox, is a classic self-reference puzzle in the philosophy of language. It arises from considering adjectives that may or may not describe themselves.

The paradox was introduced in 1908 by Kurt Grelling and Leonard Nelson. The central idea uses the term “heterological” to denote adjectives that do not describe themselves. The key question is: Is the word “heterological” heterological? If it is heterological, then it does not describe itself; but as a heterological word it would be defined as not describing itself, which seems to fit. If, on the other hand, “heterological” describes itself, then it is autological, meaning it describes itself, which contradicts the very definition of heterological. Either assignment leads to a contradiction, illustrating a problem of self-reference.

The paradox sits at the intersection of language, logic, and semantics. It shows that some seemingly straightforward predicates—such as “describes itself”—cannot be consistently applied to all words without a careful hierarchical or contextual framework. In response, logicians and philosophers developed approaches to avoid such self-reference problems, including type theory, hierarchical truth predicates, and formal semantics that separate object language from meta-language.

Grelling–Nelson has influenced later work on truth, reference, and self-reference, and is often discussed alongside the Liar paradox and Russell’s paradox as a foundational example of limits in naive semantic theories. It remains a touchstone in discussions of how language can refer to itself without collapsing into inconsistency.