Side by side with this metaphysic, Heracleitus lays down a theory of physics. All things are composed of fire. "This world," he says, "neither one of the gods nor of the human race has made; but it is, it was, and ever shall be, an eternally living fire." All comes from fire, and to fire all returns. "All things are exchanged for fire and fire for all, as wares for gold and gold for wares." Thus there is only one ultimate kind of matter, fire, and all other forms of matter are merely modifications and variations of fire. It is clear for what reason Heracleitus enunciated this principle. It is an exact physical parallel to the metaphysical principle of Becoming. Fire is the most mutable of the elements. It does not remain the same from one moment to another. It is continually taking up matter in the form of fuel, and giving off equivalent matter in the form of smoke and vapour. The primal fire, according to Heracleitus, transmutes itself into air, air into water, and water into earth. This he calls "the downward path." To it corresponds "the upward path," the transmutation of earth into water, water to air, and air to fire. All transformation takes place in this regular order, and therefore, says Heracleitus, "the upward and the downward path are one." Fire is further specially identified with life and reason. It is the rational element in things. The more fire there is, the more life, the more movement. The more dark and heavy materials there are, the more death, cold, and not-being. The soul, accordingly, is fire, and like all other fires it continually burns itself out and needs replenishment. This it obtains, through the senses and the breath, from the common life and reason of the {79} world, that is, from the surrounding and all-pervading fire. In this we live and move and have our being. No man has a separate soul of his own. It is merely part of the one universal soul-fire. Hence if communication with this is cut off, man becomes irrational and finally dies. Sleep is the half-way house to death. In sleep the passages of the senses are stopped up, and the outer fire reaches us only through breath. Hence in sleep we become irrational and senseless, turning aside from the common life of the world, each to a private world of his own. Heracleitus taught also the doctrine of periodic world-cycles. The world forms itself out of fire, and by conflagration passes back to the primitive fire. In his religious opinions Heracleitus was sceptical. But he does not, like Xenophanes, direct his attacks against the central ideas of religion, and the doctrine of the gods. He attacks mostly the outward observances and forms in which the religious spirit manifests itself. He inveighs against the worship of images, and urges the uselessness of blood sacrifice.