The following is from Philosophy of Mind: an Introduction, by Edward Feser. Hat tip: Joe Markus from the Internet Infidels Discussion board.
When you draw your mother, you are creating a kind of representation of her. But notice that it is not the particular physical features of the drawing itself - the form of the lines you make, the chemicals in the ink you use, and so forth - which make it a representation of her.........Someone looking over your shoulder as you draw might later on produce an exact copy of the drawing you were making. Perhaps the person admires your craftsmanship and wants to see if he or she can do as well. But in doing so the person would not, strictly speaking, be drawing a respresentation of your mother - he or she may have no idea, nor any interest in, who it was that you were drawing - but rather a representation of your representation. And, in general, the very same image could count either as a drawing of an X, or as a drawing of a drawing of X - or indeed (supposing there's someone looking over the shoulder of the second artist and copying what he or she was drawing) as a drawing of a drawing of a drawing of an X, and so on ad infinitum.......Even if we count something as a drawing, and therefore as possessing some intentionality or other, exactly what it is a drawing of is still indeterminate from its physical properties alone. The same is true not just of drawings, but also of written and spoken words (for to say or write "cat" could be to represent cats, but it could also be to represent the word "cat") and indeed any material representation, including purported representations encoded in neural firing patterns in the brain. There seems in general to be nothing about the physical properties of a material representation that make it a material representation of an X as opposed to a material representation of a material representation of an X.......Sometimes, however, you are determinately thinking about a particular thing or person, such as your mother. Your thought about your mother is about your mother - it represents your mother, and doesn't represent a representation of your mother (representations, pictures, and the like might be the furthest thing from your mind). But then your thought, whatever it is, cannot be entirely material. Given that there's nothing about a material representation per se that could make it a representation of an X as opposed to a representation of a representation of an X, if your thought was entirely material then there would be no fact of the matter about whether your thought represented your mother as opposed to a representation of your mother. Your thought is determinate; purely material representations are not; so your thought is not purely material.