Consider the following statement:

"Iraq has a stockpile of weapons of mass destruction."

It is simple and unambiguous. Now, compare this first sentence with the one that follows:

"George Bush says that Iraq has a stockpile of weapons of mass destruction."

Unless you've been unconscious for the past year, you'll understand that the second statement isn't really simple or unambiguous at all. In fact, quite a lot of the meaning of the second statement has nothing to do with the statement itself, but the fact that it was a statement made by George Bush (I'll make every attempt to keep this example 'bipartisan' - I use it only because it is timely and a good example).

More precisely, the second sentence is two statements, one nested inside the other and this complicates things to some degree. In the first statement, it's either true or not true. The second sentence requires two truth tests - one can say, "yes, it's true that George Bush says that Iraq has a stockpile of weapons of mass destruction, but Iraq doesn't have WMD." Likewise, the inverse can be true. The degree to which one thinks George Bush is trustworthy drives the degree to which one believes that Iraq does, in fact, have stockpiles of WMD.

From a practical perspective, it's important for us to know the source of statements and much of the applicability of RDF is in expressing explicitly the links between statements, and those who made the statements. RDF is a series of statements about statements.

RDF has a name for making statements about statements. It's called reification.