The use-mention distinction is a very important concept that comes out of the study of Logic that helps us understand the complexity and flexibility inherent in natural language. In ordinary usage, speakers may alternate between using and mentioning things without even noticing it. For example, consider the following two sentences:

Life is a many splendoured thing.
Life begins with the twelfth letter of the alphabet.

The first sentence uses the word "life", while the second sentence mentions it. When one uses a word, one invokes its meaning, intending it to stand for the thing in the "real world" that it refers to. Therefore in the first sentence, the speaker intends the listener to interpret the word "life" as referring to the concept of life as it exists for us in the world.1 In contrast to this, we would find the second sentence to be utterly non-sensical if we were to assume the speaker were using the word "life", just as in the first sentence. If so, this would mean that life began not with conception or birth (or even at forty), but rather with an L, which just seems plain strange. It is clear that in this case, the speaker is mentioning the word, intending the listener to be aware of the word itself --the lexical entity-- rather than what it stands for.

While the ability to move back and forth so easily between these two senses is convenient, it actually belies something very fundamental about the nature of natural language and its descriptive power. When we use a word, we are using language as an object language, a language which solely talks about the world. It can talk about entities in this world, and assign truth values to them. When a word is mentioned, however, we are using language to talk about itself. In the second sentence of the example, English is talking about the orthography of one of its words. This self-referential capability is a property of a metalanguage. What is so fascinating is that we are able to discuss both the world and our own manner of discourse (language) with the same language!

In typical writing, words that are mentioned are placed in quotation marks, in order to make it clear that we mean to refer to the word itself. 2 According to this convention, the second sentence of the example should really be written:

"Life" begins with the twelfth letter of the alphabet

More examples include: and so on... Note that the song "Friend is a Four Letter Word" by Cake should actually be titled "'Friend' is a Four Letter Word" because it is mentioning rather than using the word "friend".

Outside of logic and proper writing, the use-mention distinction has found an interesting place in the study of computer science. Consider the case of a bit of code that prints something out to the screen. Taking an example from C, the typical syntax for this would be:

printf("Hello World!");

where the quotation marks are used to delimit what is to be outputted. But what if we wanted to print out a phrase that itself contained quotation marks? Instead of using the quotation marks, we should like to mention them in our phrase. How does the computer know the difference between these two functions? The answer is to be found with escape characters which are are a conventional way to signal that we wish to mention the quotation marks.

printf("Why does every single textbook program write out \"Hello World\"?  Arrgh!");
Consider next the more complicated task of having a program document itself, essentially to print out its own contents. Any simple, straightforward attempt at this will fail because we inevitably have trouble switching the code from a use function to a mention function. (for more on this, see quine) What about if we wish to write self modifying code? The Java Reflection package, for example, allows programs to introspect upon themselves, to in a sense access their own code. Dizzy yet?

Finally to end this discussion on a fun note, here's something amusing that I found written in the Linguistics dept. lounge at the University of Edinburgh:

If you use a mention
in use then you
don't use the use-mention
distinction usefully.
          (just thought I should mention it)

1 This example is a bit tricky, because life isn't a very concrete concept, but bear with me.
2 For an example of this convention, just look at this writeup itself. If I refer to the word "life" (like right now!) I use quotation marks.