And of the organs they first contrived the eyes to give light, and the principle according to which they were inserted was as follows: So much of fire as would not burn, but gave a gentle light, they formed into a substance akin to the light of every-day life; and the pure fire which is within us and related thereto they made to flow through the eyes in a stream smooth and dense, compressing the whole eye, and especially the centre part, so that it kept out everything of a coarser nature, and allowed to pass only this pure element. When the light of day surrounds the stream of vision, then like falls upon like, and they coalesce, and one body is formed by natural affinity in the line of vision, wherever the light that falls from within meets with an external object. And the whole stream of vision, being similarly affected in virtue of similarity, diffuses the motions of what it touches or what touches it over the whole body, until they reach the soul, causing that perception which we call sight. But when night comes on and the external and kindred fire departs, then the stream of vision is cut off; for going forth to an unlike element it is changed and extinguished, being no longer of one nature with the surrounding atmosphere which is now deprived of fire: and so the eye no longer sees, and we feel disposed to sleep. For when the eyelids, which the gods invented for the preservation of sight, are closed, they keep in the internal fire; and the power of the fire diffuses and equalises the inward motions; when they are equalised, there is rest, and when the rest is profound, sleep comes over us scarce disturbed by dreams; but where the greater motions still remain, of whatever nature and in whatever locality, they engender corresponding visions in dreams, which are remembered by us when we are awake and in the external world. And now there is no longer any difficulty in understanding the creation of images in mirrors and all smooth and bright surfaces. For from the communion of the internal and external fires, and again from the union of them and their numerous transformations when they meet in the mirror, all these appearances of necessity arise, when the fire from the face coalesces with the fire from the eye on the bright and smooth surface. 

τῶν δὲ ὀργάνων πρῶτον μὲν φωσφόρα συνετεκτήναντο ὄμματα͵ τοιᾷδε ἐνδήσαντες αἰτίᾳ. τοῦ πυρὸς ὅσον τὸ μὲν κάειν οὐκ ἔσχε͵ τὸ δὲ παρέχειν φῶς ἥμερον͵ οἰκεῖον ἑκάστης ἡμέρας͵ σῶμα ἐμηχανήσαντο γίγνεσθαι. τὸ γὰρ ἐντὸς ἡμῶν ἀδελφὸν ὂν τούτου πῦρ εἰλικρινὲς ἐποίησαν διὰ τῶν ὀμμάτων ῥεῖν λεῖον καὶ πυκνὸν ὅλον μέν͵ μάλιστα δὲ τὸ μέσον συμπιλήσαντες τῶν ὀμμάτων͵ [45c] ὥστε τὸ μὲν ἄλλο ὅσον παχύτερον στέγειν πᾶν͵ τὸ τοιοῦτον δὲ μόνον αὐτὸ καθαρὸν διηθεῖν. ὅταν οὖν μεθημερινὸν ᾖ φῶς περὶ τὸ τῆς ὄψεως ῥεῦμα͵ τότε ἐκπῖπτον ὅμοιον πρὸς ὅμοιον͵ συμπαγὲς γενόμενον͵ ἓν σῶμα οἰκειωθὲν συνέστη κατὰ τὴν τῶν ὀμμάτων εὐθυωρίαν͵ ὅπῃπερ ἂν ἀντερείδῃ τὸ προσπῖπτον ἔνδοθεν πρὸς ὃ τῶν ἔξω συνέπεσεν. ὁμοιοπαθὲς δὴ δι΄ ὁμοιότητα πᾶν γενόμενον͵ [45d] ὅτου τε ἂν αὐτό ποτε ἐφάπτηται καὶ ὃ ἂν ἄλλο ἐκείνου͵ τούτων τὰς κινήσεις διαδιδὸν εἰς ἅπαν τὸ σῶμα μέχρι τῆς ψυχῆς αἴσθησιν παρέσχετο ταύτην ᾗ δὴ ὁρᾶν φαμεν. ἀπελθόντος δὲ εἰς νύκτα τοῦ συγγενοῦς πυρὸς ἀποτέτμηται· πρὸς γὰρ ἀνόμοιον ἐξιὸν ἀλλοιοῦταί τε αὐτὸ καὶ κατασβέννυται͵ συμφυὲς οὐκέτι τῷ πλησίον ἀέρι γιγνόμενον͵ ἅτε πῦρ οὐκ ἔχοντι. παύεταί τε οὖν ὁρῶν͵ ἔτι τε ἐπαγωγὸν ὕπνου γίγνεται· σωτηρίαν γὰρ ἣν οἱ θεοὶ τῆς ὄψεως ἐμηχανήσαντο͵ τὴν τῶν βλεφάρων φύσιν͵ [45e] ὅταν ταῦτα συμμύσῃ͵ καθείργνυσι τὴν τοῦ πυρὸς ἐντὸς δύναμιν͵ ἡ δὲ διαχεῖ τε καὶ ὁμαλύνει τὰς ἐντὸς κινήσεις͵ ὁμαλυνθεισῶν δὲ ἡσυχία γίγνεται͵ γενομένης δὲ πολλῆς μὲν ἡσυχίας βραχυόνειρος ὕπνος ἐμπίπτει͵ καταλειφθεισῶν δέ τινων κινήσεων μειζόνων͵ οἷαι καὶ ἐν οἵοις ἂν τόποις λείπωνται͵ [46a] τοιαῦτα καὶ τοσαῦτα παρέσχοντο ἀφομοιωθέντα ἐντὸς ἔξω τε ἐγερθεῖσιν ἀπομνημονευόμενα φαντάσματα. τὸ δὲ περὶ τὴν τῶν κατόπτρων εἰδωλοποιίαν καὶ πάντα ὅσα ἐμφανῆ καὶ λεῖα͵ κατιδεῖν οὐδὲν ἔτι χαλεπόν. ἐκ γὰρ τῆς ἐντὸς ἐκτός τε τοῦ πυρὸς ἑκατέρου κοινωνίας ἀλλήλοις͵ ἑνός τε αὖ περὶ τὴν λειότητα ἑκάστοτε γενομένου καὶ πολλαχῇ μεταρρυθμισθέντος͵ [46b] πάντα τὰ τοιαῦτα ἐξ ἀνάγκης ἐμφαίνεται͵ τοῦ περὶ τὸ πρόσωπον πυρὸς τῷ περὶ τὴν ὄψιν πυρὶ περὶ τὸ λεῖον καὶ λαμπρὸν συμπαγοῦς γιγνομένου.