the thought of later philosophers and theologians and began the open struggle with polytheism which was to continue for many centuries. Another philosopher who deserves our attention here is Heraclitus of Ephesus who flourished in the early fifth century. As he surveyed the world he was impressed with the variety of phenomena that moved before him, with the fact that nothing is stable but that everything is always in process of change. He declared therefore that nothing is permanent but that all things in reality are in a state of flux and flow (πάντα ῥεῖ). The explanation for this constant change he apparently found in the crude science of the day which observed that changes in temperature cause changes of form, some solids becoming liquid and liquids gaseous. This phenomenon he thought was due to fire. Fire he regarded as universal in the cosmos—indeed, as identical with it, and he is reported as saying that this universe, which is the same for all beings, no god or mortal has made, but it has always been and is and ever will be eternal fire, which sometimes grows the brighter and again is quenched. This fire he is willing to call god; it is to him the principle which permeates the universe and causes all change within it. Such being the case, Heraclitus maintains that true knowledge is not concerned with many things but solely with the unity in the world which his teachings set forth. In his scorn for his predecessors he outdoes Xenophanes. Not only Homer and Hesiod, but Pythagoras, Xenophanes, Archilochus, and Hecateus are alike condemned. Homer and Archilochus he declares deserve to be driven from the games and flogged, such folly had they taught.[159] In his assaults on the popular religion Heraclitus did not concern himself so much with mythology as with the rites and ceremonies current in his time. He evidently attacked the representatives of the mystery religions, calling them night walkers, magi, priests of Bacchus, devotees of the wine-vat, and mystery-mongers. In another fragment he ridicules those who pray to the images of the gods, for that is as if one were to pray to a man’s house; and again he declares that if men on ordinary occasions sang songs like the hymns which they raise in honor of Dionysus they would be acting most shamelessly. Man’s duty to his mind is to devote himself wholly to the apprehension of the divine unity in the world, of that wisdom to which alone he would consent to give the name of Zeus.[160]