natural things and of beings as such, then the essence of movement is needful of discussion.
Aristotle carried out this discussion in his great lectures on 'physics'. This latter word is not to be taken as equivalent to the modern concept of physics, but not for the reason that Aristotle's physics is primitive and proceeds without mathematics. It is because Aristotle's physics is not natural science at all but rather philosophy, i.e. philosophical knowledge of the φύσει ὄντα, knowledge of present things as present. Aristotelian physics is not only not more primitive than modern physics, but it is the latter's necessary presupposition, both substantively and historically.
The thematic discussion of movement occurs in the third, fifth, and eighth book of the Physics. The first book has an introductory character. Aristotle exhibits the inner necessity of the problem of movement by showing how the primary and ultimate problematic of all previous philosophy presses toward this problem. In this connection he discusses the difficulties which face any new treatment of movement. Many things about movement (the essence of movement) are problematized. Aristotle inquires into the origin of movement in its intrinsic nature. He calls that which determines the inner possibility of something the ἀρχή, principle. The fundamental nature of movement is μεταβολή, change. This is change from ... to ... If, for example, this piece of chalk for some reason (γένεσις) becomes red, we can take this in two ways: as a change from white-coloured to red-coloured, or as a becoming-red of the chalk. In the latter case white does not become red, but the white piece of chalk becomes a red piece of chalk, not just a τόδε γίνεσθαι (τόδε) ἀλλὰ καὶ ἐκ τοῦδε ...3; it does not happen that a red thing originates from the chalk. A third principle belongs to the inner possibility of the γένεσις ἔκ τινος εἴς τι the ὑπμένον, i.e. what stays the same throughout the change. But this, the chalk, a singular thing, has a twofold εἶδος: first its being-chalk, which does not necessarily involve being-white, and secondly this being-white itself. These must be different if change is to be possible, namely change as a going-over to something different to and absent from the initial state, στέρεσις. So γένεσις in the proper sense involves these three principles: 1. ὑπμένον, 2. εἶδος, 3. στέρεσις. 2 and 3 refer to the ἐναντία. For καὶ δῆλόν ἐστιν ὅτι δεῖ ὑποκεῖσθαι τι τοῖς ἐναντίοις καὶ τἀναντία δύο εἶναι.4 Thus three ἀρχαί: on the one hand ὑπμένον, on the other hand the indicated
3 Aristotle, Physics 190 a 6. ('We say not only "this becomes so-and-so", but also "from being this, comes to be so-and-so"', trans. Hardie and Gaye.)
4 Physics 191 a 4 f. ('It is clear that there must be a substratum for the contraries, and that the contraries must be two'; trans. Hardie and Gaye.)
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