For, claiming that, besides the existent, nothing non existent exists, he thinks that the existent is of necessity one and that nothing else exists (on this we have spoken more clearly in our work on nature), but being forced to follow the phenomena, and supposing that what is one in formula but many according to perception, he now posits two causes and two principles, calling them hot and cold, ie fire and earth; and of these he ranges the hot with the existent, and the other with the non existent.