The use–mention distinction is a foundational concept of analytic philosophy, according to which it is necessary to make a distinction between using a word (or phrase) and mentioning it, and many philosophical works have been "vitiated by a failure to distinguish use and mention". The distinction is disputed by non-analytic philosophers.
The first sentence is a statement about the substance called "cheese"; it uses the word 'cheese' to refer to that substance. The second is a statement about the word 'cheese' as a signifier; it mentions the word without using it to refer to anything other than itself.
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In written language, mentioned words or phrases often appear between quotation marks (as in "'Chicago' contains three vowels") or in italics (as in "When I say honey, I mean the sweet stuff that bees make"), and style authorities such as Strunk and White insist that mentioned words or phrases must always be made visually distinct in this manner. Used words or phrases (much more common than mentioned ones) do not bear any typographic distinction. In spoken language, or in absence of the use of stylistic cues such as quotation marks or italics in written language, the audience must identify mentioned words or phrases through semantic and pragmatic cues.
If quotation marks are used, it is sometimes the practice to distinguish between the quotation marks used for speech and those used for mentioned words, with double quotes in one place and single in the other:
A few authorities recommend against such a distinction, and prefer one style of quotation mark to be used for both purposes.
The general phenomenon of a term's having different references in different contexts was called suppositio (substitution) by medieval logicians. It describes how one has to substitute a term in a sentence based on its meaning—that is, based on the term's referent. In general, a term can be used in several ways. For nouns, they are:
The last sentence contains a mention example.
The use–mention distinction is especially important in analytic philosophy. Failure to properly distinguish use from mention can produce false, misleading, or meaningless statements or category errors. For example, the following correctly distinguish between use and mention:
The first sentence, a mention example, is a statement about the word "copper" and not the chemical element. Notably, the word is composed of six letters, but not any kind of metal or other tangible thing. The second sentence, a use example, is a statement about the chemical element copper and not the word itself. Notably, the element is composed of 29 electrons and protons and a number of neutrons, but not any letters.
Stanisław Leśniewski was perhaps the first to make widespread use of this distinction and the fallacy that arises from overlooking it, seeing it all around in analytic philosophy of the time, for example in Russell and Whitehead's Principia Mathematica. At the logical level, a use–mention mistake occurs when two heterogeneous levels of meaning or context are confused inadvertently.
Donald Davidson told that in his student years, "quotation was usually introduced as a somewhat shady device, and the introduction was accompanied by a stern sermon on the sin of confusing the use and mention of expressions". He presented a class of sentences like
Quine said that "quotation has a certain anomalous feature."
which both use the meaning of the quoted words to complete the sentence, and mention them as they are attributed to W. V. Quine, to argue against his teachers' hard distinction. His claim was that quotations could not be analyzed as simple expressions that mention their content by means of naming it or describing its parts, as sentences like the above would lose their exact, twofold meaning.
Self-referential statements mention themselves or their components, often producing logical paradoxes, such as Quine's paradox. A mathematical analogy of self-referential statements lies at the core of Gödel's incompleteness theorem (diagonal lemma). There are many examples of self-reference and use–mention distinction in the works of Douglas Hofstadter, who makes the distinction thus:
When a word is used to refer to something, it is said to be being used. When a word is quoted, though, so that someone is examining it for its surface aspects (typographical, phonetic, etc.), it is said to be being mentioned.
Although the standard notation for mentioning a term in philosophy and logic is to put the term in quotation marks, issues arise when the mention is itself of a mention. Notating using italics might require a potentially infinite number of typefaces, while putting quotation marks within quotation marks may lead to ambiguity.
Some analytic philosophers have said the distinction "may seem rather pedantic".
In this 1995 paper, Boolos discussed ambiguities in using quotation marks as part of a formal language, and proposed a way of distinguishing levels of mentioning using a finite number of marks, using "′" to modify the succeeding "°", as in:
According to W. Quine,
Whose views on quotation are fine,
°Boston° names Boston,
and ′°°Boston°′° names °Boston°,
But 9 doesn't designate 9.
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