Light and its vehicle are distinct
Basil is explaining the fourth day of creation, arguing that the sun and moon are not the light itself but vehicles prepared for the first-created light.
For just as fire is one thing and a lamp another—the one possessing the power to give light, the other made to shine for those who need it—so also the lights were now prepared as a vehicle for that purest and unmixed and immaterial light. For as the apostle says, there are certain lights in the world (Philippians 2:15), but the true light of the world is something else (John 1:9), and by sharing in it the saints became lights for the souls they taught, rescuing them from the darkness of ignorance. In the same way, the Maker of all things fitted this sun to that most radiant light and kindled it around the world.
And let no one think what has been said is unbelievable—that the brightness of light is one thing, and the body underlying the light is another. First, then, from the fact that all composite things are divided by us in this way, into the receptive substance and the quality that has come upon it. Just as whiteness is one thing by nature and the whitened body another, so also the things now spoken of, being different by nature, have been united by the power of the Creator. And do not tell me that it is impossible for these to be divided from one another. For I do not claim that the division of light from the solar body is possible for you and me, but that what is separable for us in thought can also be separated in actual operation by the Maker of their nature. Since even for you it is impossible to separate the burning power of fire from its brightness; yet God, wishing to turn his servant by a marvelous sight, placed fire on the bush that was active from its brightness alone, while its power to burn remained idle (Exodus 3:2). As the psalmist also testifies when he says, “The voice of the Lord divides the flame of fire” (Psalm 28:7 LXX). From this also, in the repayment of the lives we have lived, a certain teaching instructs us in mystery: that the nature of fire will be divided, and the light will be assigned for the enjoyment of the righteous, while the painful part of burning will be appointed for those being punished.
Then, moreover, from the phenomena concerning the moon, it is possible for us to find faith in what we are seeking. For when it wanes and decreases, it is not consumed in its whole body, but by putting off the light that surrounds it and taking it on again, it presents to us the appearances of decrease and increase. And that its body itself is not consumed when it wanes has clear testimony in what is seen. For you can, in clear air free from all mist, especially when it happens to be crescent-shaped, observe and see its unlit and unilluminated part outlined by an arc as large as the one that fills the whole of it at full moon. So the completed circle is clearly seen, with the eye bringing together the shadowy and misty hollow with the illuminated part. And do not tell me that the moon's light is imported, because it decreases as it moves toward the sun and increases again as it draws away. For that is not what we are examining at present, but rather that its body is one thing, and what gives it light is another.
Greek
Ὡς γὰρ ἄλλο τὸ πῦρ, καὶ ἄλλο ὁ λύχνος· τὸ μὲν τὴν τοῦ φωτίζειν δύναμιν ἔχον, τὸ δὲ παραφαίνειν τοῖς δεομένοις πεποιημένον· οὕτω καὶ τῷ καθαρωτάτῳ ἐκείνῳ καὶ εἰλικρινεῖ καὶ ἀΰλῳ φωτὶ ὄχημα νῦν οἱ φωστῆρες κατεσκευάσθησαν. Ὡς γὰρ ὁ ἀπόστολος λέγει τινὰς φωστῆρας ἐν κόσμῳ, ἄλλο δέ ἐστι φῶς τοῦ κόσμου τὸ ἀληθινὸν, οὗ κατὰ μέθεξιν οἱ ἅγιοι φωστῆρες ἐγίνοντο τῶν ψυχῶν, ἃς ἐπαίδευον, τοῦ σκότους αὐτὰς τῆς ἀγνοίας ῥυόμενοι· οὕτω καὶ νῦν τὸν ἥλιον τοῦτον τῷ φανοτάτῳ ἐκείνῳ ἐπισκευάσας φωτὶ ὁ τῶν ὅλων δημιουργὸς περὶ τὸν κόσμον ἀνῆψε. 6.3 Καὶ μηδενὶ ἄπιστον εἶναι δοκείτω τὸ εἰρημένον, ὅτι ἄλλο μέν τι τοῦ φωτὸς ἡ λαμπρότης, ἄλλο δέ τι τὸ ὑποκεί μενον τῷ φωτὶ σῶμα. Πρῶτον μὲν οὖν ἐκ τοῦ τὰ σύνθετα πάντα οὕτω παρ' ἡμῶν διαιρεῖσθαι, εἴς τε τὴν δεκτικὴν οὐσίαν, καὶ εἰς τὴν ἐπισυμβᾶσαν αὐτῇ ποιότητα. Ὡς οὖν ἕτερον μέν τι τῇ φύσει ἡ λευκότης, ἕτερον δέ τι τὸ λελευκασμένον σῶμα, οὕτω καὶ τὰ νῦν εἰρημένα, διάφορα ὄντα τῇ φύσει, ἥνωται τῇ δυνάμει τοῦ κτίσαντος. Καὶ μή μοι λέγε ἀδύνατα εἶναι ταῦτα ἀπ' ἀλλήλων διαιρεῖσθαι. Οὐδὲ γὰρ ἐγὼ τὴν διαίρεσιν τοῦ φωτὸς ἀπὸ τοῦ ἡλιακοῦ σώματος ἐμοὶ καὶ σοὶ δυνατὴν εἶναί φημι, ἀλλ' ὅτι ἃ ἡμῖν τῇ ἐπινοίᾳ ἐστὶ χωριστὰ, ταῦτα δύναται καὶ αὐτῇ τῇ ἐνεργείᾳ παρὰ τοῦ ποιητοῦ τῆς φύσεως αὐτῶν διαστῆναι. Ἐπεὶ καὶ σοὶ τὴν καυστικὴν δύναμιν τοῦ πυρὸς ἀπὸ τῆς λαμπρότητος χωρίσαι ἀμήχανον· ὁ δὲ Θεὸς παραδόξῳ θεάματι τὸν ἑαυτοῦ θερά ποντα ἐπιστρέψαι βουλόμενος, πῦρ ἐπέθηκε τῇ βάτῳ ἀπὸ μόνης τῆς λαμπρότητος ἐνεργοῦν, τὴν δὲ τοῦ καίειν δύναμιν σχολάζουσαν ἔχον. Ὡς καὶ ὁ ψαλμῳδὸς μαρτυρεῖ λέ γων, Φωνὴ Κυρίου διακόπτοντος φλόγα πυρός. Ὅθεν καὶ ἐν ταῖς τῶν βεβιωμένων ἡμῖν ἀνταποδόσεσι λόγος τις ἡμᾶς ἐν ἀπορρήτῳ παιδεύει, διαιρεθήσεσθαι τοῦ πυρὸς τὴν φύσιν, καὶ τὸ μὲν φῶς, εἰς ἀπόλαυσιν τοῖς δικαίοις, τὸ δὲ τῆς καύσεως ὀδυνηρὸν, τοῖς κολαζομένοις ἀποταχθήσεσθαι. Ἔπειτα μέντοι καὶ ἐκ τῶν περὶ σελήνην παθῶν, δυνατὸν ἡμᾶς τὴν πίστιν τῶν ζητουμένων εὕρασθαι. Λήγουσα γὰρ, καὶ μειουμένη, οὐχὶ τῷ παντὶ ἑαυτῆς σώματι δαπανᾶται, ἀλλὰ τὸ περικείμενον φῶς ἀποτιθεμένη καὶ προσλαμβάνουσα πάλιν, ἐλαττώσεως ἡμῖν καὶ αὐξήσεως τὰς φαντασίας παρέχεται. Τοῦ δὲ μὴ αὐτὸ τὸ σῶμα αὐτῆς ληγούσης ἀπαναλίσκεσθαι ἐναργὲς μαρτύριον τὰ ὁρώμενα. Ἔξεστι γάρ σοι καὶ ἐν καθαρῷ τῷ ἀέρι καὶ πάσης ἀχλύος ἀπηλλαγμένῳ, ὅταν μάλιστα μηνοειδὴς τυγχάνῃ κατὰ τὸ σχῆμα, ἐπιτηρήσαντι κατιδεῖν τὸ ἀλαμπὲς αὐτῆς καὶ ἀφώτιστον ὑπὸ τηλικαύτης ἁψῖδος περιγραφόμενον, ἡλίκον ἐν ταῖς πανσελήνοις τὴν πᾶσαν αὐτὴν ἐκπληροῖ. Ὥστε τηλαυγῶς ἀπηρτισμένον καθορᾶσθαι τὸν κύκλον τῷ περιλαμπομένῳ μέρει τὸν σκιερὸν καὶ ἀερώδη κόλπον συναναφερούσης τῆς ὄψεως. Καὶ μή μοι λέγε ἐπείσακτον εἶναι τῆς σελήνης τὸ φῶς, διότι μειοῦται μὲν πρὸς ἥλιον φερομένη, αὔξεται δὲ πάλιν ἀφισταμένη. Οὐδὲ γὰρ ἐκεῖνο ἡμῖν ἐξετάζειν ἐν τῷ παρόντι πρόκειται, ἀλλ' ὅτι ἕτερον μὲν αὐτῆς τὸ σῶμα, ἕτερον δὲ τὸ φωτίζον.