Of their physical system, however, much is known, and the reader of Epictetus needs to be acquainted with its general features. These were borrowed from an earlier thinker, Heracleitus, whose central doctrine was that the universe was an eternal flux and transition; everything was in a state of becoming, _ein Werdendes_. At the beginning of things, so far as they can be said to have any beginning, is the Deity in his purest manifestation, which, be it observed, is a strictly material one, a sublimated and ethereal fire, αἰθεῶδες πῦρ. In this fire dwelt the divine creative thought and impulse. The first step in that process of differentiation in which development consists is the production of vapor, which condensed into water. Two elementary forces play their part in these operations—a movement towards within, and a movement towards without, the one a densifying, the other an expanding and straining force (τόνος). The former gives us solidity in matter, the other the qualities and energies of matter. Thus, by various degrees of density, we get earth, water, atmospheric air, and from air, the common element of earthly fire; and these elements in their various combinations, with their various attributes and powers, gradually produce the successive stages of organic life. Though all these proceed from the substance of the Divine Being, the Stoics recognized, in the derived substance which make up the universe as we have it now, various degrees of purity, of affinity to their original source. Man’s body, for instance, with its passions and affections, lies comparatively far from the divine; but his soul is a veritable ray of the primitive fire, _Deus in corpore humano hospitans_. The popular mythology of the day was entirely rejected by the Stoics, although, as Professor Mahaffy points out, they never attempted to “discredit orthodoxy,” but, on the contrary, used its myths and ceremonies with the utmost reverence as vehicles of profound religious truths. But they certainly believed in intelligences above man, yet below the one Supreme Being; thus the stars and the lightning (the reader will observe the allusions in the Hymn of Cleanthes) are in some sense divinities, by virtue of the supposed purity of their fiery essence.