instead of this, we were to assume a number of existent things, it would be quite possible to apply all that Parmenides says of reality to each of them, and the forms of existence we know might be explained by the mingling and separation of those realities. The conception of “elements” (στοιχεῖα), to use a later term,[580] was found, and the required formula follows at once. So far as concerns particular things, it is true, as our senses tell us, that they come into being and pass away; but, if we have regard to the ultimate elements of which they are composed, we shall say with Parmenides that “what _is_” is uncreated and indestructible (fr. 17). Footnote 579: Cf. Emp. frs. 27, 28, with Parm. fr. 8. Footnote 580: For the history of the term στοιχεῖον see Diels, _Elementum_. Eudemos said (_ap._ Simpl. _Phys._ p. 7, 13) that Plato was the first to use it, and this is confirmed by the way the word is introduced in _Tht._ 201 e. The original term was μορφή or ἰδέα. [Sidenote: The “four roots.”] 107. The “four roots” of all things (fr. 6) which Empedokles assumed were those that have become traditional—Fire, Air, Earth, and Water. It is to be noticed, however, that he does not call Air ἀήρ, but αἰθήρ, and this must be because he wished to avoid any confusion with what had hitherto been meant by the former word. He had, in fact, made the great discovery that atmospheric air is a distinct corporeal substance, and is not to be identified with empty space on the one hand or rarefied mist on the other. Water is not liquid air, but something quite different.[581] This truth Empedokles demonstrated by means of the apparatus known as the _klepsydra_, and we still possess the verses in which he applied his discovery to the explanation of respiration and the motion of the blood (fr. 100). Aristotle laughs at those who try to show there is no empty space by shutting up air in water-clocks and torturing wineskins. They only prove, he says, that air is a thing.[582] That, however, is exactly what Empedokles intended to prove, and it was one of the most important discoveries in the early history of science. It will be convenient for us to translate the αἰθήρ of Empedokles by “air”; but we must be careful in that case not to render the word ἀήρ in the same way. Anaxagoras seems to have been the first to use it of atmospheric air. Footnote 581: Cf. Chap. I. § 27. Footnote 582: Arist. _Phys._ Δ, 6, 213 a 22 (R. P. 159). Aristotle only mentions Anaxagoras by name in this passage; but he speaks in the plural, and we know from fr. 100 that the _klepsydra_ experiment was used by Empedokles.