Aristotle, Poetics

Aristot. Poet. 1450b

κεῖται δὴ ἡμῖν τὴν τραγῳδίαν τελείας καὶ ὅλης πράξεως εἶναι [25] μίμησιν ἐχούσης τι μέγεθος: ἔστιν γὰρ ὅλον καὶ μηδὲν ἔχον μέγεθος. ὅλον δέ ἐστιν τὸ ἔχον ἀρχὴν καὶ μέσον καὶ τελευτήν. ἀρχὴ δέ ἐστιν ὃ αὐτὸ μὲν μὴ ἐξ ἀνάγκης μετ᾽ ἄλλο ἐστίν, μετ᾽ ἐκεῖνο δ᾽ ἕτερον πέφυκεν εἶναι ἢ γίνεσθαι: τελευτὴ δὲ τοὐναντίον ὃ αὐτὸ μὲν μετ᾽ ἄλλο πέφυκεν εἶναι ἢ [30] ἐξ ἀνάγκης ἢ ὡς ἐπὶ τὸ πολύ, μετὰ δὲ τοῦτο ἄλλο οὐδέν: μέσον δὲ ὃ καὶ αὐτὸ μετ᾽ ἄλλο καὶ μετ᾽ ἐκεῖνο ἕτερον. δεῖ ἄρα τοὺς συνεστῶτας εὖ μύθους μήθ᾽ ὁπόθεν ἔτυχεν ἄρχεσθαι μήθ᾽ ὅπου ἔτυχε τελευτᾶν, ἀλλὰ κεχρῆσθαι ταῖς εἰρημέναις ἰδέαις.

After these definitions we must next discuss the proper arrangement of the incidents since this is the first and most important thing in tragedy. We have laid it down that tragedy is a representation of an action that is whole and complete and of a certain magnitude, since a thing may be a whole and yet have no magnitude. A whole is what has a beginning and middle and end. A beginning is that which is not a necessary consequent of anything else but after which something else exists or happens as a natural result. An end on the contrary is that which is inevitably or, as a rule, the natural result of something else but from which nothing else follows; a middle follows something else and something follows from it. Well constructed plots must not therefore begin and end at random, but must embody the formulae we have stated.

Aristotle. Aristotle in 23 Volumes, Vol. 23, translated by W.H. Fyfe. Cambridge, MA, Harvard University Press; London, William Heinemann Ltd. 1932.

http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.01.0056:section%3D1450b