In considering the third kind of sense, hearing, we must speak of the causes in which it originates. We may in general assume sound to be a blow which passes through the ears, and is transmitted by means of the air, the brain, and the blood, to the soul, and that hearing is the vibration of this blow, which begins in the head and ends in the region of the liver. The sound which moves swiftly is acute, and the sound which moves slowly is grave, and that which is regular is equable and smooth, and the reverse is harsh. A great body of sound is loud, and a small body of sound the reverse. Respecting the harmonies of sound I must hereafter speak.

There is a fourth class of sensible things, having many intricate varieties, which must now be distinguished. They are called by the general name of colours, and are a flame which emanates from every sort of body, and has particles corresponding to the sense of sight. I have spoken already, in what has preceded, of the causes which generate sight, and in this place it will be natural and suitable to give a rational theory of colours.

Of the particles coming from other bodies which fall upon the sight, some are smaller and some are larger, and some are equal to the parts of the sight itself. Those which are equal are imperceptible, and we call them transparent. The larger produce contraction, the smaller dilation, in the sight, exercising a power akin to that of hot and cold bodies on the flesh, or of astringent bodies on the tongue, or of those heating bodies which we termed pungent. White and black are similar effects of contraction and dilation in another sphere, and for this reason have a different appearance. Wherefore, we ought to term white that which dilates the visual ray, and the opposite of this is black. There is also a swifter motion of a different sort of fire which strikes and dilates the ray of sight until it reaches the eyes, forcing a way through their passages and melting them, and eliciting from them a union of fire and water which we call tears, being itself an opposite fire which comes to them from an opposite direction-the inner fire flashes forth like lightning, and the outer finds a way in and is extinguished in the moisture, and all sorts of colours are generated by the mixture. This affection is termed dazzling, and the object which produces it is called bright and flashing.

[67b] Τρίτον δὲ αἰσθητικὸν ἐν ἡμῖν μέρος ἐπισκοποῦσιν τὸ περὶ τὴν ἀκοήν͵ δι΄ ἃς αἰτίας τὰ περὶ αὐτὸ συμβαίνει παθήματα͵ λεκτέον. ὅλως μὲν οὖν φωνὴν θῶμεν τὴν δι΄ ὤτων ὑπ΄ ἀέρος ἐγκεφάλου τε καὶ αἵματος μέχρι ψυχῆς πληγὴν διαδιδομένην͵ τὴν δὲ ὑπ΄ αὐτῆς κίνησιν͵ ἀπὸ τῆς κεφαλῆς μὲν ἀρχομένην͵ τελευτῶσαν δὲ περὶ τὴν τοῦ ἥπατος ἕδραν͵ ἀκοήν· ὅση δ΄ αὐτῆς ταχεῖα͵ ὀξεῖαν͵ ὅση δὲ βραδυτέρα͵ βαρυτέραν· τὴν δὲ ὁμοίαν ὁμαλήν τε καὶ λείαν͵ τὴν δὲ ἐναντίαν τραχεῖαν· [67c] μεγάλην δὲ τὴν πολλήν͵ ὅση δὲ ἐναντία͵ σμικράν. τὰ δὲ περὶ συμφωνίας αὐτῶν ἐν τοῖς ὕστερον λεχθησομένοις ἀνάγκη ῥηθῆναι.

Τέταρτον δὴ λοιπὸν ἔτι γένος ἡμῖν αἰσθητικόν͵ ὃ διελέσθαι δεῖ συχνὰ ἐν ἑαυτῷ ποικίλματα κεκτημένον͵ ἃ σύμπαντα μὲν χρόας ἐκαλέσαμεν͵ φλόγα τῶν σωμάτων ἑκάστων ἀπορρέουσαν͵ ὄψει σύμμετρα μόρια ἔχουσαν πρὸς αἴσθησιν· ὄψεως δ΄ ἐν τοῖς πρόσθεν αὐτὸ περὶ τῶν αἰτίων τῆς γενέσεως ἐρρήθη. [67d] τῇδ΄ οὖν τῶν χρωμάτων πέρι μάλιστα εἰκὸς πρέποι τ΄ ἂν ἐπιεικεῖ λόγῳ διεξελθεῖν·

τὰ φερόμενα ἀπὸ τῶν ἄλλων μόρια ἐμπίπτοντά τε εἰς τὴν ὄψιν τὰ μὲν ἐλάττω͵ τὰ δὲ μείζω͵ τὰ δ΄ ἴσα τοῖς αὐτῆς τῆς ὄψεως μέρεσιν εἶναι· τὰ μὲν οὖν ἴσα ἀναίσθητα͵ ἃ δὴ καὶ διαφανῆ λέγομεν͵ τὰ δὲ μείζω καὶ ἐλάττω͵ τὰ μὲν συγκρίνοντα͵ τὰ δὲ διακρίνοντα αὐτήν͵ τοῖς περὶ τὴν σάρκα θερμοῖς καὶ ψυχροῖς καὶ τοῖς περὶ τὴν γλῶτταν στρυφνοῖς͵ [67e] καὶ ὅσα θερμαντικὰ ὄντα δριμέα ἐκαλέσαμεν͵ ἀδελφὰ εἶναι͵ τά τε λευκὰ καὶ τὰ μέλανα͵ ἐκείνων παθήματα γεγονότα ἐν ἄλλῳ γένει τὰ αὐτά͵ φανταζόμενα δὲ ἄλλα διὰ ταύτας τὰς αἰτίας. οὕτως οὖν αὐτὰ προσρητέον· τὸ μὲν διακριτικὸν τῆς ὄψεως λευκόν͵ τὸ δ΄ ἐναντίον αὐτοῦ μέλαν͵ τὴν δὲ ὀξυτέραν φορὰν καὶ γένους πυρὸς ἑτέρου προσπίπτουσαν καὶ διακρίνουσαν τὴν ὄψιν μέχρι τῶν ὀμμάτων͵ αὐτάς τε τῶν ὀφθαλμῶν τὰς διεξόδους βίᾳ διωθοῦσαν καὶ τήκουσαν͵ [68a] πῦρ μὲν ἁθρόον καὶ ὕδωρ͵ ὃ δάκρυον καλοῦμεν͵ ἐκεῖθεν ἐκχέουσαν͵ αὐτὴν δὲ οὖσαν πῦρ ἐξ ἐναντίας ἀπαντῶσαν͵ καὶ τοῦ μὲν ἐκπηδῶντος πυρὸς οἷον ἀπ΄ ἀστραπῆς͵ τοῦ δ΄ εἰσιόντος καὶ περὶ τὸ νοτερὸν κατασβεννυμένου͵ παντοδαπῶν ἐν τῇ κυκήσει ταύτῃ γιγνομένων χρωμάτων͵ μαρμαρυγὰς μὲν τὸ πάθος προσείπομεν͵ τὸ δὲ τοῦτο ἀπεργαζόμενον λαμπρόν τε καὶ στίλβον ἐπωνομάσαμεν.