"His teachings formed a series of poems some five thousand verses in length. Only a hundred and fifty verses have survived from... On Nature yet, the relics are more substantial than those from any other Greek philosopher. From them we can extract a theory which... tackles all three problems of Greek science. ...(a) What are the stable principles behind the flux? (b) What process is responsible for the changes in the flux? (c) What agencies control this process? To these questions Empedokles replied... (a) The enduring principles in the natural world are the four basic types of matter—solid, liquid, fiery and aeriform. ...they are conserved in all material transformations. (b) Change comes about through the mingling and separation of these... which unite in different proportions to produce... familiar objects... (c) The agents responsible... are the two universal powers acting in opposition, which he called allegorically, Love and Strife. ...[T]his [as an explicit theory] was the first appearance in our scientific tradition of an important intellectual model. ...[A]ll material things are organized mixtures of different elementary substances ...And, as developed by his contemporary Anaxagoras, and later by the atomists, this type of matter-theory has been in circulation ever since."