Now we have seen that a generation is the shortest time in which a man can become a grandfather, it is the period of the upward or downward path of the soul, and the most natural interpretation of the longer period would surely be that it represents the time taken by a “measure” of the fire in the world to travel on the downward path to earth or return to fire once more by the upward path. Plato certainly implies that such a parallelism between the periods of man and the world was recognised,[400] and this receives a curious confirmation from a passage in Aristotle, which is usually supposed to refer to the doctrine of a periodic conflagration. He is discussing the question whether the “heavens,” that is to say, what he calls the “first heaven,” is eternal or not, and he naturally enough, from his own point of view, identifies this with the Fire of Herakleitos. He quotes him along with Empedokles as holding that the “heavens” are alternately as they are now and in some other state, one of passing away; and he goes on to point out that this is not really to say they pass away, any more than it would be to say that a man ceases to be, if we said that he turned from boy to man and then from man to boy again.[401] It is surely clear that this is a reference to the parallel between the generation and the Great Year, and, if so, the ordinary interpretation of the passage must be wrong. It is true that it is not quite consistent with the theory to suppose that a “measure” of Fire could preserve its identity throughout the whole of its upward and downward path; but it is exactly the same inconsistency that we have felt bound to recognise with regard to the continuance of individual souls, a fact which is really in favour of our interpretation. It should be added that, while 18,000 is half 36,000, 10,800 is 360 × 30, which would make each generation a day in the Great Year.[402] Footnote 400: This is certainly the general sense of the parallelism between the periods of the ἀνθρώπειον and the θεῖον γεννητόν, however we may understand the details. See Adam, _Republic_, vol. ii. pp. 288 sqq. Footnote 401: Arist. _de Caelo_, Α, 10. 279 b 14, οἱ δ’ ἐναλλὰξ ὁτὲ μὲν οὕτως ὁτὲ δὲ ἄλλως ἔχειν φθειρόμενον, ... ὥσπερ Ἐμπεδοκλῆς ὀ Ἀκραγαντῖνος καὶ Ἡράκλειτος ὁ Ἐφέσιος. Aristotle points out that this really amounts only to saying that it is eternal and changes its form, ὥσπερ εἴ τις ἐκ παιδὸς ἄνδρα γιγνόμενον καὶ ἐξ ἀνδρὸς παῖδα ὁτὲ μὲν φθείρεσθαι, ὁτὲ δ’ εἶναι οἴοιτο (280 a 14). The point of the reference to Empedokles will appear from _de Gen. Corr._ Β, 6. 334 a 1 sqq. What Aristotle finds fault with in both theories is that they do not regard the substance of the heavens as something outside the upward and downward motion of the elements. Footnote 402: This is practically Lassalle’s view of the Great Year, except that he commits the anachronism of speaking of “atoms” of fire instead of “measures.”