The Atomists had assumed the ultimate constituents of things to be atoms composed of the same kind of matter. Empedocles had believed in four ultimate and underived kinds of matter. With neither of these does Anaxagoras agree. For him, all the different kinds of {96} matter are equally ultimate and underived, that is to say, such things as gold, bone, hair, earth, water, wood, etc., are ultimate kinds of matter, which do not arise from anything else, and do not pass over into one another. He also disagrees with the conception of the Atomists that if matter is divided far enough, ultimate and indivisible particles will be reached. According to Anaxagoras matter is infinitely divisible. In the beginning all these kinds of matter were mixed together in a chaotic mass. The mass stretches infinitely throughout space. The different kinds of matter wholly intermingle and interpenetrate each other. The process of world-formation is brought about by the unmixing of the conglomeration of all kinds of matter, and the bringing together of like matter with like. Thus the gold particles separating out of the mass come together, and form gold; the wood particles come together and form wood, and so on. But as matter is infinitely divisible and the original mixing of the elements was complete, they were, so to speak, mixed to an infinite extent. Therefore the process of unmixing would take infinite time, is now going on, and will always go on. Even in the purest element there is still a certain admixture of particles of other kinds of matter. There is no such thing as pure gold. Gold is merely matter in which the gold particles predominate.