Quine's paradox
Quine's paradox is a paradox concerning truth values, attributed to Willard Van Orman Quine. It is related to the liar paradox as a problem, and it purports to show that a sentence can be paradoxical even if it is not self-referring and does not use demonstratives or indexicals (i.e. it does not explicitly refer to itself). The paradox can be expressed as follows:
If the paradox is not clear, consider each part of the above description of the paradox incrementally:
With these tools, the description of the paradox may now be reconsidered; it can be seen to assert the following:
In other words, the sentence implies that it is false, which is paradoxical—for if it is false, what it states is in fact true.
The liar paradox ("This sentence is false", or "The next sentence is true. The previous sentence is false") demonstrates essential difficulties in assigning a truth value even to simple sentences. Many philosophers, attempting to explain the liar paradox, concluded that the problem was with the use of demonstrative word "this" or its replacements. Once we properly analyze this sort of self-reference, according to said philosophers, the paradox no longer arises.
Quine's construction demonstrates that paradox of this kind arises independently of such direct self-reference, for, no lexeme of the sentence refers to the sentence, though Quine's sentence does contain a lexeme which refers to one of its parts. Namely, "its" near the end of the sentence is a possessive pronoun whose antecedent is the very predicate in which it occurs. Thus, although Quine's sentence per se is not self-referring, it does contain a self-referring predicate.
Any system, such as English, that contains entities such as words or sentences that can be used to apply to themselves, must contain this type of paradox. There is no way to eliminate the paradoxes, short of a severe crippling of the language.
Quine suggested an unnatural linguistic resolution to such logical antinomies, inspired by Bertrand Russell's Type theory and Tarski's work. His system would attach levels to a line of problematic expressions such as falsehood and denote. Entire sentences would stand higher in the hierarchy than their parts. The form "'Clause about falsehood0' yields falsehood1" will be grammatically correct, and "'Denoting0 phrase' denotes0 itself" – wrong.
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