A definition of the nature of complexity, Kadanoff says, "has been somewhat elusive." But if one were to try, "what we see is a world in which there seems to be organization built up in some rich and interesting fashion--from huge mountain ranges, to the delicate ridge on the surface of a sand dune, to the salt spray coming off a wave, to the interdependencies of financial markets, to the true ecologies formed by living things. For each kind of organization, we want to understand how it arose and whether it has any general rules associated with it."