 Philosophers of mind often divide the problems of physicalism into two: first, there are the problems of qualia, typified by the knowledge argument; second, there are problems of intentionality. The intentionality of mental states is their aboutness, their capacity to represent the world as being a certain way. One does not simply think, one thinks of (or about) Vienna; similarly, one does not simply believe, one believes that snow is white. Just as in the case of qualia, some of the puzzles of intentionality derive from facts internal to the notion, and from the relation of this notion to the others such as rationality, inference and language; see  Intentionality.  But others derive from the fact that it seems difficult to square the fact that mental states have intentionality with physicalism. There are a number of ways of developing this criticism but much recent work has concentrated on a certain line of argument that Saul Kripke has found in the work of Wittgenstein (1982; see also  Private Language).