_Heraclitus_ was an Ephesian, and is said to have been a pupil of Xenophanes, though this statement is much doubted; others call him a pupil of Hippasus the Pythagorean. He wrote a treatise on Nature; declaring that the principle of all things was fire, from which he saw the world was evolved by a natural operation; he further said that this fire was the human life and soul, and therefore a rational intelligence guiding the whole universe. In this primary fire he considered that there was a perpetual longing to manifest itself in different forms: in its perfectly pure state it is in heaven; but in order to gratify this longing it descends, gradually losing the rapidity of its motion till it settles in the earth. The earth, however, is not immovable, but only the slowest of all moving bodies; while the soul of man, though dwelling in the lowest of all regions, namely, in the earth, he considered a migrated portion of fire in its pure state; which, in spite of its descent, had lost none of its original purity. The _summum bonum_ he considered to be a contented acquiescence in the decrees of the Deity. None of his writings are extant; and he does not appear to have had many followers. _Diogenes_ of Apollonia, (who must not be confounded with his Stoic or Cynic namesake,) was a pupil of Anaximenes, and wrote a treatise on Nature, of which Diogenes Laertius gives the following account: “He maintained that air was the primary element of all things; that there was an infinite number of worlds and an infinite vacuum; that air condensed and rarefied produced the different members of the universe; that nothing was generated from nothing, or resolved into nothing; that the earth was round, supported in the centre, having received its shape from the whirling round it of warm vapours, and its concrete nature and hardness from cold.” He also imputed to air an intellectual energy, though he did not recognise any difference between mind and matter.