NLESS a person comes to an understanding about the nature and conditions of rest and motion, he will meet with many difficulties in the discussion which follows. Something has been said of this matter already, and something more remains to be said, which is, that motion never exists in what is uniform. For to conceive that anything can be moved without a mover is hard or indeed impossible, and equally impossible to conceive that there can be a mover unless there be something which can be moved-motion cannot exist where either of these are wanting, and for these to be uniform is impossible; wherefore we must assign rest to uniformity and motion to the want of uniformity. Now inequality is the cause of the nature which is wanting in uniformity; and of this we have already described the origin. But there still remains the further point-why things when divided after their kinds do not cease to pass through one another and to change their place-which we will now proceed to explain. In the revolution of the universe are comprehended all the four elements, and this being circular and having a tendency to come together, compresses everything and will not allow any place to be left void. Wherefore, also, fire above all things penetrates everywhere, and air next, as being next in rarity of the elements; and the two other elements in like manner penetrate according to their degrees of rarity. For those things which are composed of the largest particles have the largest void left in their compositions, and those which are composed of the smallest particles have the least. And the contraction caused by the compression thrusts the smaller particles into the interstices of the larger. And thus, when the small parts are placed side by side with the larger, and the lesser divide the greater and the greater unite the lesser, all the elements are borne up and down and hither and thither towards their own places; for the change in the size of each changes its position in space. And these causes generate an inequality which is always maintained, and is continually creating a perpetual motion of the elements in all time. 

κίνησις καὶ στάσις

Κινήσεως οὖν στάσεώς τε πέρι͵ τίνα τρόπον καὶ μεθ΄ ὧντινων γίγνεσθον͵ εἰ μή τις διομολογήσεται͵ [57e] πόλλ΄ ἂν εἴη ἐμποδὼν τῷ κατόπισθεν λογισμῷ. τὰ μὲν οὖν ἤδη περὶ αὐτῶν εἴρηται͵ πρὸς δ΄ ἐκείνοις ἔτι τάδε͵ ἐν μὲν ὁμαλότητι μηδέποτε ἐθέλειν κίνησιν ἐνεῖναι. τὸ γὰρ κινησόμενον ἄνευ τοῦ κινήσοντος ἢ τὸ κινῆσον ἄνευ τοῦ κινησομένου χαλεπόν͵ μᾶλλον δὲ ἀδύνατον͵ εἶναι· κίνησις δὲ οὐκ ἔστιν τούτων ἀπόντων͵ ταῦτα δὲ ὁμαλὰ εἶναί ποτε ἀδύνατον. οὕτω δὴ στάσιν μὲν ἐν ὁμαλότητι͵ κίνησιν δὲ εἰς ἀνωμαλότητα ἀεὶ τιθῶμεν· [58a] αἰτία δὲ ἀνισότης αὖ τῆς ἀνωμάλου φύσεως. ἀνισότητος δὲ γένεσιν μὲν διεληλύθαμεν· πῶς δέ ποτε οὐ κατὰ γένη διαχωρισθέντα ἕκαστα πέπαυται τῆς δι΄ ἀλλήλων κινήσεως καὶ φορᾶς͵ οὐκ εἴπομεν. ὧδε οὖν πάλιν ἐροῦμεν. ἡ τοῦ παντὸς περίοδος͵ ἐπειδὴ συμπεριέλαβεν τὰ γένη͵ κυκλοτερὴς οὖσα καὶ πρὸς αὑτὴν πεφυκυῖα βούλεσθαι συνιέναι͵ σφίγγει πάντα καὶ κενὴν χώραν οὐδεμίαν ἐᾷ λείπεσθαι. [58b] διὸ δὴ πῦρ μὲν εἰς ἅπαντα διελήλυθε μάλιστα͵ ἀὴρ δὲ δεύτερον͵ ὡς λεπτότητι δεύτερον ἔφυ͵ καὶ τἆλλα ταύτῃ· τὰ γὰρ ἐκ μεγίστων μερῶν γεγονότα μεγίστην κενότητα ἐν τῇ συστάσει παραλέλοιπεν͵ τὰ δὲ σμικρότατα ἐλαχίστην. ἡ δὴ τῆς πιλήσεως σύνοδος τὰ σμικρὰ εἰς τὰ τῶν μεγάλων διάκενα συνωθεῖ. σμικρῶν οὖν παρὰ μεγάλα τιθεμένων καὶ τῶν ἐλαττόνων τὰ μείζονα διακρινόντων͵ τῶν δὲ μειζόνων ἐκεῖνα συγκρινόντων͵ πάντ΄ ἄνω κάτω μεταφέρεται πρὸς τοὺς ἑαυτῶν τόπους· [58c] μεταβάλλον γὰρ τὸ μέγεθος ἕκαστον καὶ τὴν τόπων μεταβάλλει στάσιν. οὕτω δὴ διὰ ταῦτά τε ἡ τῆς ἀνωμαλότητος διασῳζομένη γένεσις ἀεὶ τὴν ἀεὶ κίνησιν τούτων οὖσαν ἐσομένην τε ἐνδελεχῶς παρέχεται.