Plut. _def. orac._ 415 d, ἔτη τριάκοντα ποιοῦσι τὴν γενεὰν καθ’ Ἡράκλειτον, ἐν ᾧ χρόνῳ γεννῶντα παρέχει τὸν ἐξ αὑτοῦ γεγεννημένον ὁ γεννήσας. Philo, fr. Harris, p. 20, δυνατὸν ἐν τριακοστῷ ἔτει αὖ τὸν ἄνθρωπον πάππον γενέσθαι κ.τ.λ. Censorinus, _de die nat._ 17, 2, “hoc enim tempus (triaginta annos) _genean_ vocari Heraclitus auctor est, quia _orbis aetatis_ in eo sit spatio: orbem autem vocat aetatis, dum natura ab sementi humana ad sementim revertitur.” The words _orbis aetatis_ seem to mean αἰῶνος κύκλος, “the circle of life.” If so, we may compare the Orphic κύκλος γενέσεως. [Sidenote: The day and the year.] 76. Let us turn now to the world. Diogenes tells us that fire was kept up by the bright vapours from land and sea, and moisture by the dark.[394] What are these “dark” vapours which increase the moist element? If we remember the “Air” of Anaximenes, we shall be inclined to regard them as darkness itself. We know that the idea of darkness as privation of light is not natural to the unsophisticated mind. We sometimes hear even now of darkness “thick enough to cut with a knife.” I suppose, then, that Herakleitos believed night and winter to be produced by the rise of darkness from earth and sea—he saw, of course, that the valleys were dark before the hill-tops,—and that this darkness, being moist, so increased the watery element as to put out the sun’s light. This, however, destroys the power of darkness itself. It can no longer rise upwards unless the sun gives it motion, and so it becomes possible for a fresh sun (fr. 32) to be kindled, and to nourish itself at the expense of the moist element for a time. But it can only be for a time. The sun, by burning up the bright vapour, deprives himself of nourishment, and the dark vapour once more gets the upper hand. It is in this sense that “day and night are one” (fr. 35). Each implies the other, and they are therefore to be regarded as merely two sides of the one, in which alone their true ground of explanation is to be found (fr. 36). Footnote 394: Diog. ix. 9 (R. P. 39 b). Summer and winter were easily to be explained in the same way. We know that the “turnings” of the sun were a subject of interest in those days, and it was natural for Herakleitos to see in its retreat further to the south the gradual advance of the moist element, caused by the heat of the sun itself. This, however, diminishes the power of the sun to cause evaporation, and so it must return to the north once more that it may supply itself with nourishment. Such was, at any rate, the Stoic doctrine on the subject,[395] and that it comes from Herakleitos seems to be proved by its occurrence in the Περὶ διαίτης. It seems impossible to refer the following sentence to any other source:—