- Man consists of three letters.
- Man is a rational animal.
- Some animal consists of three letters.
St. Augustine was aware of the difficulty, but his solution was, at least on its surface, different. He posited that the word "man" (or at least "homo") has two meanings. One of its meanings is something like rational animal, while the other meaning is something like the word consisting of the sequence of letters M, A and N. We have a systematic ambiguity running through our language: each word, in addition to its dictionary meaning, also means itself. (An interesting consequence which Augustine I think did not note is that each verb is also a noun.)
We could imagine accepting Augustine's solution, and then when necessary marking the distinction by the use of subscripts: man1 is a rational animal and man2 has three letters.
But perhaps we have accepted Augustine's solution? Instead of subscripting a "2", we superscript a quotation mark before and after when we mean the word, and instead of subscripting a "1", we leave the word unadorned. Thus, man is an animal, but "man" is not.
So, is there any difference between Augustine's proposal and the use-mention distinction? Maybe this much: Identifying the concept of the mention of a bit of language saves us from multiplying our vocabulary indefinitely by having to introduce the infinite set of nouns "'man'", "'a man sits'", "'a man sits or floats'", "'a man sits or floats or sits'", and so on.
But since "'a man sits'" surely is a noun denoting "a man sits", we still have introduced an infinite number of nouns. However, we did so by means of a quotation-functor which systematically takes any bit of language and returns a new bit of language denoting that bit of language. So all that the use-mention distinction comes down to is that sometimes bits of language are in the scope of a quotation-functor and sometimes they're not. And it is an accident of the quotation-functor of English, both spoken and written, that the sounds of the denoted bit of language are contained in the noun denoting the bit of language, and this accident we describe by talking of the "use-mention distinction". We could have had a different kind of quotation-functor, for instance using the Goedel number of a bit of language in golden ink to denote that bit of language, and forbidding the use of golden ink for anything else. And then it wouldn't make much sense to talk of use-mention. So the fundamental phenomenon here isn't use-mention, but meta-language.