W. T. Stace, A critical history of Greek philosophy
“ 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. ”
