 One potential problem with possible-worlds contents concerns experiences that represent impossible scenes. Consider the staircase depicted in the Escher drawing, on which one seems to be able to make four consecutive left turns, ending where one begins, yet walking downstairs (or upstairs) the whole time. Another example of contradictory contents comes from the waterfall illusion, when one looks at a stationary object after looking at a waterfall, the object appears to move, yet appears to stay still relative to its background. Similar illusions of movement can be achieved by looking at spirals on paper. These experiences differ phenomenally from one another. Moreover, although all seem to represent both that something is moving and that the very same thing is not moving, they represent much more specific scenes as well, and these specifics clearly differ. It seems that possible-worlds contents could not reflect these differences. Since the set of possible of worlds in which things are as the experience represents them to be is the null set, the view that says that the only contents of experience are possible worlds would posit the same content (i.e., the null set) for all of them.