Grelling–Nelson paradox

The Grelling–Nelson paradox is a semantic self-referential paradox formulated in 1908 by Kurt Grelling and Leonard Nelson and sometimes mistakenly attributed to the German philosopher and mathematician Hermann Weyl. It is thus occasionally called Weyl's paradox as well as Grelling's paradox. It is closely analogous to several other well-known paradoxes, in particular the Barber paradox and Russell's paradox.

The paradox
Suppose one interprets the adjectives "autological" and "heterological" as follows:
 * 1) An adjective is autological (sometimes homological) if and only if it describes itself. For example "short" is autological, since the word "short" is short. "Sophisticated" and "pentasyllabic" are also autological.
 * 2) An adjective is heterological if and only if it does not describe itself. Hence "long" is a heterological word, as is "monosyllabic".

All adjectives, it would seem, must be either autological or heterological, for each adjective either describes itself, or it doesn't. The Grelling–Nelson paradox arises when we consider the adjective "heterological".

To test if the (imaginary) word "'foo" is autological one can ask: Is "foo" a foo word? If the answer is 'yes', "foo" is autological. If the answer is 'no', "foo" is heterological.

By comparison, one can ask: Is "heterological" a heterological word? If the answer is 'yes', "heterological" is autological (leading to a contradiction). If the answer is 'no', "heterological" is heterological (again leading to a contradiction).

The paradox can be eliminated, without changing the meaning of "heterological" where it was previously well-defined, by modifying the definition of "heterological" slightly to hold of all nonautological words except "heterological." But "nonautological" is subject to the same paradox, for which this evasion is not applicable because the rules of English uniquely determine its meaning from that of "autological." A similar slight modification to the definition of "autological" (such as declaring it false of "nonautological" and its synonyms) might seem to fix that, but the paradox still obtains for synonyms of "autological" and "heterological" such as "selfdescriptive" and "nonselfdescriptive," whose meanings also would need adjusting, and the consequences of those adjustments would then need to be pursued, and so on. Freeing English of the Grelling–Nelson paradox entails considerably more modification to the language than mere refinements of the definitions of "autological" and "heterological," which need not even be in the language for the paradox to arise. The scope of these obstacles for English is comparable to that of Russell's paradox for mathematics founded on sets, argued as follows.

Similarities with Russell's paradox
The Grelling–Nelson paradox can be translated into Bertrand Russell's famous paradox in the following way. First one must identify each adjective with the set of objects to which that adjective applies. So, for example, the adjective "red" is equated with the set of all red objects. In this way, the adjective "pronounceable" is equated with the set of all pronounceable things, one of which is the word "pronounceable" itself. Thus, an autological word is understood as a set, one of whose elements is the set itself. The question of whether the word "heterological" is heterological becomes the question of whether the set of all sets not containing themselves contains itself as an element.