We have now seen from these two representatives how philosophy regarded traditional polytheism, and taught that the unity of the universe was identical with god. But neither Xenophanes nor Heraclitus offered any satisfactory explanation of the way in which multiplicity could arise out of unity. Still less did they conceive of a personal god. This concept was reserved for Plato or possibly for his great teacher. A slight approach toward an explanation of variety was made, it is true, by Heraclitus, through his “fire” and also through a doctrine which I have not mentioned, namely that “strife,” the action of opposites, is the cause of change. Now another philosopher, Empedocles of Agrigentum, whom we have already met as a mystic, offered an explanation of the cause of phenomena not dissimilar to that of Heraclitus, and yet one that marked an advance. For his elements Empedocles took the four of popular belief—earth, air, fire, water, which he said were combined in various forms by the principles of “friendship” and of “strife”; or, as we should say today in less symbolic language, by affinity and incompatibility. But like Heraclitus Empedocles fails to make clear how or why these principles act at all on his elements. In short before Anaxagoras no thinker conceived of any satisfactory formative or motive principles to explain phenomena; likewise none had arrived at a well defined distinction between a material principle and a formative principle. Anaxagoras solved the problem in a way highly satisfactory for his age and in a manner which unconsciously anticipated many of the principles of modern science. According to him all substances are elementary, existing in seeds or germs, infinite in number, infinitesimal in size. Aristotle and the ancients in general understood him to mean that these seeds or germs are minute particles of the things which we know in the mass, as for example bread or water, or flesh and bones. Some modern scholars think that he meant that the original mass of matter was infinitely divided and that every atom had in it a portion of everything else: the various combinations of these seeds, atoms, we name bread, water, or flesh and bones according to the predominance of these things in the seeds which make up the whole. Yet whether Aristotle and the ancients or the moderns be right is a matter of little moment to us now. Anaxagoras thought that these “seeds,” whatever they were, were set in motion and combined to produce the infinite diversity of the natural objects by Mind (Νοῦς).