Early Greek Philosophy by John Burnet, with Burnet's notes
178. The Eternal Motion 180. The Vortex

From Chapter IX., Leukippos of Miletos

179. The Weight of the Atoms
As is well known, Epicurus held that the atoms were naturally heavy, and therefore fell continually in the infinite void. The school tradition is, however, that the "natural weight" of the atoms was an addition made by Epicurus himself to the original atomic system. Demokritos, we are told, assigned two properties to atoms, magnitude and form, to which Epicurus added a third, weight.33 On the other hand, Aristotle distinctly says that Demokritos held the atoms were heavier "in proportion to their excess," and this seems to be explained by the statement of Theophrastos that, according to him, weight depended on magnitude.34 Even so, however, it is not represented as a primary property of the atoms in the same sense as magnitude.

It is impossible to solve this apparent contradiction without referring briefly to the history of Greek ideas about weight. It is clear that lightness and weight would be among the very first properties of body to be distinctly recognised as such. The necessity of lifting burdens must very soon have led men to distinguish them, though no doubt in a crude form. Both weight and lightness would be thought of as things that were in bodies. Now it is a remarkable feature of early Greek philosophy that from the first it was able to shake itself free from this idea. Weight is never called a "thing" as, for instance, warmth and cold are; and, so far as we can see, not one of the thinkers we have studied hitherto thought it necessary to give any explanation of it at all, or even to say anything about it.35 The motions and resistances which popular theory ascribes to weight are all explained in some other way. Aristotle distinctly declares that none of his predecessors had said anything of absolute weight and lightness. They had only treated of the relatively light and heavy.36

This way of regarding the notions of weight and lightness is clearly formulated for the first time in Plato's Timaeus.37 There is no such thing in the world, we are told there, as "up" or "down." The middle of the world is not "down" but "just in the middle," and there is no reason why any point in the circumference should be said to be "above" or "below" another. It is really the tendency of bodies towards their kin that makes us call a falling body heavy and the place to which it falls "below." Here Plato is really giving the view taken more or less consciously by his predecessors, and it is not till the time of Aristotle that it is questioned.38 For reasons which do not concern us here, Aristotle identified the circumference of the heavens with "up" and the middle of the world with "down," and equipped the elements with natural weight and lightness that they might perform their rectilinear motions between them. As, however, Aristotle believed there was only one world, and did not ascribe weight to the heavens proper, the effect of this reactionary theory on his cosmical system was not great; it was only when Epicurus tried to combine it with the infinite void that its true character emerged. It seems to me that the nightmare of Epicurean atomism can only be explained on the assumption that an Aristotelian doctrine was violently adapted to a theory which really excluded it.39 It is totally unlike anything we meet with in earlier days.

This suggests at once that it is only in the vortex that the atoms acquire weight and lightness,40 which are, after all, only popular names for facts which can be further analysed. We are told that Leukippos held one effect of the vortex to be that like atoms were brought together with their likes.41 Here we seem to see the influence of Empedokles, though the "likeness" is of another kind. It is the finer atoms that are forced to the circumference, while the larger tend to the centre. We may express that by saying that the larger are heavy and the smaller light, and this will amply account for everything Aristotle and Theophrastos say; for there is no passage where the atoms outside the vortex are distinctly said to be heavy or light.42

There is a striking confirmation of this view in the atomist cosmology quoted above.43 We are told there that the separation of the larger and smaller atoms was due to the fact that they were "no longer able to revolve in equilibrium owing to their number," which implies that they had previously been in a state of "equilibrium" or "equipoise." Now the word ἰσορροπία has no necessary implication of weight in Greek. A ῥοπή is a mere leaning or inclination in a certain direction, which is the cause rather than the effect of weight. The state of ἰσορροπία is therefore that in which the tendency in one direction is exactly equal to the tendency in any other, and such a state is more naturally described as the absence of weight than as the presence of opposite weights neutralising one another.

Now, if we no longer regard the "eternal motion" of the premundane and extramundane atoms as due to their weight, there is no reason for describing it as a fall. None of our authorities do as a matter of fact so describe it, nor do they tell us in any way what it was. It is safest to say that it is simply a confused motion this way and that.44 It is possible that the comparison of the motion of the atoms of the soul to that of the motes in a sunbeam coming through a window, which Aristotle attributes to Demokritos,45 is really intended as an illustration of the original motion of the atoms still surviving in the soul. The fact that it is also a Pythagorean comparison46 so far confirms this; for we have seen that there is a real connexion between the Pythagorean monads and the atoms. It is also significant that the point of the comparison appears to have been the fact that the motes in the sunbeam move even when there is no wind, so that it would be a very apt illustration indeed of the motion inherent in the atoms apart from the secondary motions produced by impact and collision.

Burnet's Notes


33. Aet. i. 3, 18 (of Epicurus), συμβεβηκέναι δὲ τοῖς σώμασι τρία ταῦτα, σχῆμα, μέγεθος, βάρος. Δημόκριτος μὲν γὰρ ἔλεγε δύο, μέγεθός τε καὶ σχῆμα, ὁ δὲ Ἐπίκουρος τούτοις καὶ τρίτον βάρος προσέθηκεν· ἀνάγκη γάρ, φησί, κινεῖσθαι, τὰ σώματα τῇ τοῦ βάρους πληγῇ· ἐπεὶ οὐ κινηθήσεται; ib. 12, 6, Δημόκριτος τὰ πρῶτά φησι σώματα, ταῦτα δ' ἦν τὰ ναστά, βάρος μὲν οὐκ ἔχειν, κινεῖσθαι δὲ κατ' ἀλληλοτυπίαν ἐν τῷ ἀπείρῳ. Cic. De fato, 20, " vim motus habebant (atomi) a Democrito impulsionis quam plagam ille appellat, a te, Epicure, gravitatis et ponderis." These passages represent the Epicurean school tradition, which would hardly misrepresent Demokritos on so important a point. His works were still accessible. It is confirmed by the Academic tradition in De fin. i. 17 that Demokritos taught the atoms moved "in infinito inani, in quo nihil nec summum nec infimum nec medium nec extremum sit." This doctrine, we are quite rightly told, was "depraved" by Epicurus.

34. Arist. De gen. corr. A, 8. 326 a 9, καίτοι βαρύτερόν γε κατὰ τὴν ὑπεροχήν φησιν εἶναι Δημόκριτος ἕκαστον τῶν ἀδιαιρέτων. I cannot believe this means anything else than what Theophrastos says in his fragment on sensation, § 67 (R. P. 199), βαρὺ μὲν οὖν καὶ κοῦφον τῷ μεγέθει διαιρεῖ Δημόκριτος.

35. In Aet. i. 12, where the placita regarding the heavy and light are given, no philosopher earlier than Plato is referred to. Parmenides (fr. 8, 59) speaks of the dark element as ἐμβριθές. Empedokles (fr. 17) uses the word ἀτάλαντον. I do not think that there is any other place where weight is even mentioned in the fragments of the early philosophers.

36. Arist. De caelo, Δ, I. 3o8 a 9, περὶ μὲν οὖν τῶν ἁπλῶς λεγομένων (βαρέων καὶ κούφων) οὐδὲν εἴρηται παρὰ τῶν πρότερον.

37. Plato, Tim. 61 c 3 sqq.

38. Zeller says (p. 876) that in antiquity no one ever understood by weight anything else than the property of bodies in virtue of which they move downwards; except that in such systems as represent all forms of matter as contained in a sphere, "above" is identified with the circumference and "below" with the centre. As to that, I can only say that no such theory of weight is to be found in the fragments of the early philosophers or is anywhere ascribed to them, while Plato expressly denies it.

39. The Aristotelian criticisms which may have affected Epicurus are such as we find in De caelo, A, 7. 275 b 29 sqq. Aristotle there argues that, as Leukippos and Demokritos made the φύσις of the atoms one, they were bound to give them a single motion. That is just what Epicurus did, but Aristotle's argument implies that Leukippos and Demokritos did not. Though he gave the atoms weight, even Epicurus could not accept Aristotle's view that some bodies are naturally light. The appearance of lightness is due to ἔκθλιψις the squeezing out of the smaller atoms by the larger.

40. In dealing with Empedokles, Aristotle expressly makes this distinction. Cf. De caelo, B, 13, especially 295 a 32 sqq., where he points out that Empedokles does not account for the weight of bodies on the earth (οὐ γὰρ ἥ γε δίνη πλησιάζει πρὸς ἡμᾶς), nor for the weight of bodies before the vortex arose (πρὶν γενέσθαι τὴν δίνην).

41. Diog. loc. cit. (p. 338).

42. This seems to be in the main the view of Dyroff, Demokritstudien (1899), pp. 31 sqq., though I should not say that lightness and weight only arose in connexion with the atoms of the earth (p. 35), If we substitute "world" for "earth," we shall be nearer the truth.

43. See above, p. 338.

44. This view was independently advocated by Brieger (Die Urbewegung der Atome und die Weltentstehung bei Leucipp and Demokrit, 1884) and Liepmann (Die Mechanik der Leucipp-Demokritschen Atome, 1885), both of whom unnecessarily weakened their position by admitting that weight is an original property of the atoms. On the other hand, Brieger denies that the weight of the atoms is the cause of their original motion, while Liepmann says that before and outside the vortex there is only a latent weight, a Pseudoschwere, which only comes into operation in the world. It is surely simpler to say that this weight, since it produces no effect, does not yet exist. Zeller rightly argues against Brieger and Liepmann that, if the atoms have weight, they must fall; but, so far as I can see, nothing he says tells against their theory as I have restated it. Gomperz adopts the Brieger-Liepmann explanation. See also Lortzing, Bursians Jahresber., 1903, pp. 136 sqq.

45. Arist. De an. A, 2. 403 b 28 sqq. (R. P. 200).

46. Ibid. A, 2, 404 a 17 (R. P. 86 a).

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