 Some possible worlds are easier to contemplate negatively. Thales said that all is water. Suppose he was nearly right except for the existence of two bubbles. These two absences of water become the interesting players (just as two drops of water in an otherwise empty space become interesting players in the dual of this universe). How would these bubbles relate to each other? Would the bubbles repel each other? Would the bubbles be mutually unaffected? Deep thinking about gravity yields the conclusion that the bubbles would attract each other! (Epstein 1983, 138–9)