Weight and lightness
Preliminary definitions. The terms “heavy” and “light” may be used absolutely as well as relatively. In the former sense they mean moving naturally to the centre and the cir-
307 b28Περὶ δὲ βαρέος καὶ κούφου, τί τ᾿ ἐστὶν ἑκάτερον καὶ τίς ἡ φύσις αὐτῶν, σκεπτέον, καὶ διὰ τίν᾿ 30αἰτίαν ἔχουσι τὰς δυνάμεις ταύτας. ἔστι γὰρ ἡ περὶ αὐτῶν θεωρία τοῖς περὶ κινήσεως λόγοις οἰκεία· βαρὺ γὰρ καὶ κοῦφον τῷ δύνασθαι κινεῖσθαι φυσικῶς πως λέγομεν. (ταῖς δὲ ἐνεργείαις ὀνόματ᾿ αὐτῶν οὐ κεῖται, πλὴν εἴ τις οἴοιτο τὴν ῥοπὴν εἶναι 308 aτοιοῦτον.) διὰ δὲ τὸ τὴν φυσικὴν μὲν εἶναι πραγματείαν περὶ κινήσεως, ταῦτα δ᾿ ἔχειν ἐν ἑαυτοῖς οἷον ζώπυρ᾿ ἄττα κινήσεως, πάντες μὲν χρῶνται ταῖς δυνάμεσιν αὐτῶν, οὐ μὴν διωρίκασί γε, πλὴν ὀλίγων. ἰδόντες οὖν πρῶτον τὰ παρὰ τῶν ἄλλων 5εἰρημένα, καὶ διαπορήσαντες ὅσα πρὸς τὴν σκέψιν ταύτην διελεῖν ἀναγκαῖον, οὕτω καὶ τὸ φαινόμενον ἡμῖν εἴπωμεν περὶ αὐτῶν.
Weight and Lightness
cumference of the world respectively. If “up” and “down” are applied to these motions, their use is legitimate, and it is wrong to deny (with Plato and others) their application to the world as a whole.
Our next topic must be weight and lightness. What are they, what is their nature, and why have they their particular powers? This is an inquiry relevant to a study of motion, for in calling things heavy and light we mean that they have a capacity for a certain natural motion. (There are no names for the realization of these capacities, unless we count “momentum” as one.) And because physical science is concerned with motion, and weight and lightness contain within themselves, as one might say, the germs of motion, all investigators have availed themselves of their powers, but none save a few have defined them. Let us then first see what others have said, then state the difficulties whose recognition is demanded by the subject, and so reach an explanation of our own view.