Victor Reppert: What is it about one physical state that makes it about another physical state? That's the question being dealt with here.What is it about a mental state that makes it about a physical state?
Suppose you think of a tree. What makes that thought about a tree?
It seems to me that you know it is about a tree because your thought of a tree is a mental model of past experiences or potential experiences of a tree.
A tree usually has a trunk, roots, branches and leaves and so does my mental model. Of course, I can alter my model of a tree and imagine a tree, say, without roots. But what makes a mental model about an actual tree is my ability to recognize the corresponding tree if I saw it.
In general, it's difficult to model a thing that we would not recognize. The model always has at least some recognizable properties. For example, I can conceive of the kidnapper of the Lindbergh baby. Though I may not initially know what the kidnapper physically looks like, I do know what experiences relating to a person would lead me to recognize him as the kidnapper.
So, if we look at a computer (as [the previous commenter] suggests), we can see that the computer's manipulation of Shakespeare fails to be about the plays or the subject of the plays because 1) the computer is not dealing in a model of the subject of the plays, and 2) the computer is (presently) incapable of recognizing what the model of the plays is supposed to represent. The computer has only a stored representation of the play. It has no experiences, nor software for modeling those experiences, so it's file containing the play isn't about the subject of the play.
However, none of this precludes us creating a computer that can model its experiences, and can recognize the implications of its models. We might have to give the computer a corporeal existence (or a simulated corporeal existence) before it will understand what the plays are actually about, but it could be done.