70. This necessarily brings with it a certain way of looking at the change and movement of the world. Fire burns continuously and without interruption. It is therefore always consuming fuel and always liberating smoke. Everything is either mounting upwards to serve as fuel, or sinking downwards after having nourished the flame. It follows that the whole of reality is like an ever-flowing stream, and that nothing is ever at rest for a moment. The substance of the things we see is in constant change. Even as we look at them, some of the matter of which they are composed has already passed into something else, while fresh matter has come into them from another source. This theory is usually summed up, appropriately enough, in the phrase “All things are flowing” (πάντα ῥεῖ), though, as it happens, it cannot be proved that this is a quotation from Herakleitos. Plato, however, expresses the idea quite clearly. “Nothing ever is, everything is becoming”; “All things are in motion like streams”; “All things are passing, and nothing abides”; “Herakleitos says somewhere that all things pass and naught abides; and, comparing things to the current of a river, he says that you cannot step twice into the same stream” (cf. fr. 41)—these are the terms in which he describes the system. And Aristotle says the same thing, “All things are in motion,” “nothing steadfastly is.”[375] Herakleitos held, in fact, that any given thing, however stable in appearance, was merely a section in the stream, and that the matter composing it was never the same in any two consecutive moments of time. We shall see presently how he conceived this process to operate; meanwhile we remark that the idea was not altogether novel, and that it is hardly the central point in the system of Herakleitos. The Milesians held a similar view. The flux of Herakleitos was at most more unceasing and universal. Footnote 375: Plato, _Tht._ 152 e 1; _Crat._ 401 d 5, 402 a 8; Arist. _Top._ Α, 11. 104 b 22; _de Caelo_, Γ, 1. 298 b 30; _Phys._ Θ, 3. 253 b 2. [Sidenote: The Upward and Downward path.] 71. Herakleitos appears to have worked out the details of the perpetual flux with reference to the theories of Anaximenes.[376] It is unlikely, however, that he explained the transformations of matter by means of rarefaction and condensation.[377] Theophrastos, it appears, suggested that he did; but he allowed it was by no means clear. The passage from Diogenes which we are about to quote has faithfully preserved this touch.[378] In the fragments, at any rate, we find nothing about rarefaction and condensation. The expression used is “exchange” (fr. 22); and this is certainly a very good name for what happens when fire gives out smoke and takes in fuel instead. Footnote 376: See above, Chap. I. § 29. Footnote 377: See, however, the remark of Diels quoted R. P. 36 c. Footnote 378: Diog. ix. 8, σαφῶς δ’ οὐθὲν ἐκτίθεται.