In their explanation of the universe the Stoics held to a materialism which they borrowed from the teachings of Heraclitus, who had maintained that only matter had any existence whatever; therefore their system was in theory a materialistic monism, but with their monistic principle they combined an idea which in reality they had derived from Aristotle, apparently without realizing the possible consequence to their view that matter alone exists. Although holding that everything is material, they recognized in all things the presence of an active and a passive principle, the active principle forming and directing, the other being formed and directed, so that by the operation of the active principle upon the passive all the phenomena of the world come into being. The passive principle corresponded to Aristotle’s material, while the active principle included both his efficient and final causes. To their active principle the Stoics gave all the characteristics that Heraclitus had given to his λόγος, reason, or Anaxagoras to his νοῦς, mind. In short they attributed to it all the characteristics of reason and intelligence, so that in spite of their argument that the active principle was no less material than the passive, that it was the element of fire or vapor or both, it was inevitable that in practice their philosophy should ultimately tend toward a dualism and that the ancient conflict of matter and mind, of body and soul, should have its place in their teaching. They thought that the operative principle, fire, the divine reason, expresses itself in every part of the universe, that everything which exists is permeated by this divine spirit and directed by it. It is nothing less than the world-reason, God, which begets all things; so that they called it λόγος σπερματικός, that is, the reason that contains within itself the germs of all things that are to be. Now since man is of course a part of the cosmos, the Stoic argued that in him the world-reason naturally expresses itself; it is that which guides him, in fact it is his reason, the directing portion of his soul. And it is the possession of this soul, itself a part of the universal reason, which makes it possible for man to live in accord with Nature, for he attains that aim whenever his soul is in agreement with the universal soul which is its source. In this way the Stoic, for all his materialism, emphasized the divine nature of man and the community of human reason with God.