In this post, I present the use-mention distinction. Consider the following sentences:
John told me a few words.
John told me "a few words".
Those sentences don't mean the same thing. The first one means that John told me a few words, without specifying what those words were. The second means that John told me literaly "a few words", that is, he told me exactly three words, which were "a", "few", and "words".
The distinction seems trivial in this example, but it is seldom fully respected. Consider for instance:
There's a difference between him and her.
There's a difference between "him" and "her".
The first sentence refers to two people, one male and one female, and says that they are different. The second says that the words "him" and "her" are different. This example is clear, but consider:
Notice that affect is always a verb.
Notice that "affect" is always a verb.
People will often write the first sentence without noticing that it is not what they mean, and that it is not even gramatically correct. Indeed, what about:
Notice that that is never a verb.
Notice that "that" is never a verb.
Clearly, we need the quotation marks.
Worse still, there are names and titles.
I love War and Peace.(referring to the book by Tolstoy)
I love war and peace.(referring to war and peace in general)
There are often typographical and case distinctions to disambiguate, but they are not always obvious:
I love adventure.(referring to the classical video game called "adventure")
I love adventure.(referring to adventure in general)
Here is a final example:
I read a book yesterday.(ie. I read some book)
I read "a book" yesterday.(ie. I read the literal words "a book")
I read A Book yesterday.(ie. I read this book — not an endorsement, I don't know it, but the title is fun)
The fact that humans are almost never confused by missing quotation marks or missing italics is pretty surprising when you're used to working with computers.