The bistable figure featured yesterday is also called a figure ground perception illusion. The idea here is that the mind has to somehow use visual clues to decide if the white part is the foreground against a black background (which would make the image a vase), or if the black part is the foreground against a white background (two faces). When there are no obvious clues, the mind could jump back and forth between the two alternatives. Here is another version, Alex Eylar's Good and Evil. Which do you see?