Aristotle, Poetics 1454a
“Clearly, the resolution of the plots should come from the plot itself and not, as in the Medea, from some divine contrivance or as in the Iliad during the rush to the ships (Il. 2.155). The divine device should instead be used for events that are outside the drama either for those that come before what people could know or those that come later which require prophecy and revelation—since we allow that the gods may see everything. There should be nothing illogical in the events, unless it comes from outside the tragedy itself as in Sophocles’ Oedipus.”
φανερὸν οὖν ὅτι καὶ τὰς λύσεις τῶν μύθων ἐξ αὐτοῦ δεῖ τοῦ μύθου συμβαίνειν, καὶ μὴ ὥσπερ ἐν τῇ Μηδείᾳ ἀπὸ μηχανῆς καὶ ἐν τῇ ᾿Ιλιάδι τὰ περὶ τὸν ἀπόπλουν. ἀλλὰ μηχανῇ χρηστέον ἐπὶ τὰ ἔξω τοῦ δράματος, ἢ ὅσα πρὸ τοῦ γέγονεν ἃ οὐχ οἷόν τε ἄνθρωπον εἰδέναι, ἢ ὅσα ὕστερον, ἃ δεῖται προαγορεύσεως καὶ ἀγγελίας· ἅπαντα γὰρ ἀπο-δίδομεν τοῖς θεοῖς ὁρᾶν. ἄλογον δὲ μηδὲν εἶναι ἐν τοῖς πράγμασιν, εἰ δὲ μή, ἔξω τῆς τραγῳδίας, οἷον τὸ ἐν τῷ Οἰδίποδι τῷ Σοφοκλέους.