The reason why these names are used, and the circumstances under which they are ordinarily applied by us to the division of the heavens, may be elucidated by the following supposition:-if a person were to stand in that part of the universe which is the appointed place of fire, and where there is the great mass of fire to which fiery bodies gather-if, I say, he were to ascend thither, and, having the power to do this, were to abstract particles of fire and put them in scales and weigh them, and then, raising the balance, were to draw the fire by force towards the uncongenial element of the air, it would be very evident that he could compel the smaller mass more readily than the larger; for when two things are simultaneously raised by one and the same power, the smaller body must necessarily yield to the superior power with less reluctance than the larger; and the larger body is called heavy and said to tend downwards, and the smaller body is called light and said to tend upwards. And we may detect ourselves who are upon the earth doing precisely the same thing. For we of separate earthy natures, and sometimes earth itself, and draw them into the uncongenial element of air by force and contrary to nature, both clinging to their kindred elements. But that which is smaller yields to the impulse given by us towards the dissimilar element more easily than the larger; and so we call the former light, and the place towards which it is impelled we call above, and the contrary state and place we call heavy and below respectively. Now the relations of these must necessarily vary, because the principal masses of the different elements hold opposite positions; for that which is light, heavy, below or above in one place will be found to be and become contrary and transverse and every way diverse in relation to that which is light, heavy, below or above in an opposite place. And about all of them this has to be considered:-that the tendency of each towards its kindred element makes the body which is moved heavy, and the place towards which the motion tends below, but things which have an opposite tendency we call by an opposite name. Such are the causes which we assign to these phenomena. As to the smooth and the rough, any one who sees them can explain the reason of them to another. For roughness is hardness mingled with irregularity, and smoothness is produced by the joint effect of uniformity and density.

ὅθεν δὲ ὠνομάσθη ταῦτα καὶ ἐν οἷς ὄντα εἰθίσμεθα δι΄ ἐκεῖνα καὶ τὸν οὐρανὸν ὅλον οὕτω διαιρούμενοι λέγειν͵ [63b] ταῦτα διομολογητέον ὑποθεμένοις τάδε ἡμῖν. εἴ τις ἐν τῷ τοῦ παντὸς τόπῳ καθ΄ ὃν ἡ τοῦ πυρὸς εἴληχε μάλιστα φύσις͵ οὗ καὶ πλεῖστον ἂν ἠθροισμένον εἴη πρὸς ὃ φέρεται͵ ἐπεμβὰς ἐπ΄ ἐκεῖνο καὶ δύναμιν εἰς τοῦτο ἔχων͵ μέρη τοῦ πυρὸς ἀφαιρῶν ἱσταίη τιθεὶς εἰς πλάστιγγας͵ αἴρων τὸν ζυγὸν καὶ τὸ πῦρ ἕλκων εἰς ἀνόμοιον ἀέρα βιαζόμενος δῆλον ὡς τοὔλαττόν που τοῦ μείζονος ῥᾷον βιᾶται· [63c] ῥώμῃ γὰρ μιᾷ δυοῖν ἅμα μετεωριζομένοιν τὸ μὲν ἔλαττον μᾶλλον͵ τὸ δὲ πλέον ἧττον ἀνάγκη που κατατεινόμενον συνέπεσθαι τῇ βίᾳ͵ καὶ τὸ μὲν πολὺ βαρὺ καὶ κάτω φερόμενον κληθῆναι͵ τὸ δὲ σμικρὸν ἐλαφρὸν καὶ ἄνω. ταὐτὸν δὴ τοῦτο δεῖ φωρᾶσαι δρῶντας ἡμᾶς περὶ τόνδε τὸν τόπον. ἐπὶ γὰρ γῆς βεβῶτες γεώδη γένη διιστάμενοι͵ καὶ γῆν ἐνίοτε αὐτὴν ἕλκομεν εἰς ἀνόμοιον ἀέρα βίᾳ καὶ παρὰ φύσιν͵ [63d] ἀμφότερα τοῦ συγγενοῦς ἀντεχόμενα͵ τὸ δὲ σμικρότερον ῥᾷον τοῦ μείζονος βιαζομένοις εἰς τὸ ἀνόμοιον πρότερον συνέπεται· κοῦφον οὖν αὐτὸ προσειρήκαμεν καὶ τὸν τόπον εἰς ὃν βιαζόμεθα͵ ἄνω͵ τὸ δ΄ ἐναντίον τούτοις πάθος βαρὺ καὶ κάτω. ταῦτ΄ οὖν δὴ διαφόρως ἔχειν αὐτὰ πρὸς αὑτὰ ἀνάγκη διὰ τὸ τὰ πλήθη τῶν γενῶν τόπον ἐναντίον ἄλλα ἄλλοις κατέχειν - τὸ γὰρ ἐν ἑτέρῳ κοῦφον ὂν τόπῳ τῷ κατὰ τὸν ἐναντίον τόπον ἐλαφρῷ [63e] καὶ τῷ βαρεῖ τὸ βαρὺ τῷ τε κάτω τὸ κάτω καὶ τὸ ἄνω τῷ ἄνω πάντ΄ ἐναντία καὶ πλάγια καὶ πάντως διάφορα πρὸς ἄλληλα ἀνευρεθήσεται γιγνόμενα καὶ ὄντα - τόδε γε μὴν ἕν τι διανοητέον περὶ πάντων αὐτῶν͵ ὡς ἡ μὲν πρὸς τὸ συγγενὲς ὁδὸς ἑκάστοις οὖσα βαρὺ μὲν τὸ φερόμενον ποιεῖ͵ τὸν δὲ τόπον εἰς ὃν τὸ τοιοῦτον φέρεται͵ κάτω͵ τὰ δὲ τούτοις ἔχοντα ὡς ἑτέρως θάτερα. περὶ δὴ τούτων αὖ τῶν παθημάτων ταῦτα αἴτια εἰρήσθω. λείου δ΄ αὖ καὶ τραχέος παθήματος αἰτίαν πᾶς που κατιδὼν καὶ ἑτέρῳ δυνατὸς ἂν εἴη λέγειν· σκληρότης γὰρ ἀνωμαλότητι μειχθεῖσα͵ τὸ δ΄ ὁμαλότης πυκνότητι παρέχεται.