Aristotle's account of the origin of tragedy seems on the whole quite sound. The sparseness of archaeological and other evidence has long frustrated scholars, but it seems that Aristotle's suggestion that tragedy evolved from the dithyramb is as good as any we have. Dionysus is the Greek god of vegetation and wine, and the dithyrambs in honor of him are thought to have been part of festivals celebrating the harvest and the changing of the seasons. These songs were thus part of religious ceremonies, and the speaker that accompanied the large chorus was probably a priest of some sort. Though initially improvised, these dithyrambs developed a more rigid structure, and the speaker often engaged in dialogue with the chorus. Aeschylus is generally credited with the innovation of adding a second actor, which transformed choral singing into dialogue, ritual into drama. In short, Aeschylus invented tragedy and is the first great playwright of the Western tradition.
Near the end of Chapter 5, Aristotle mentions that one of the differences between tragedy and epic poetry is that the action of a tragedy usually unfolds in the space of a single day. This is often interpreted as one of the three "unities" of tragic drama. In fact, the three unities—unity of action (one single plot with no loose threads), unity of time (action takes place within a single day), and unity of place (action takes place in a single location)—were not invented by Aristotle at all. The Italian theorist Lodovico Castelvetro formalized these unities in 1570. This formalization was inspired by the Poetics, but it is far more restrictive than anything Aristotle says. The only unity he insists upon, as we shall see, is the unity of action. His reference here to the unity of time seems to be a general guideline and not one that must be followed strictly, and there is even less evidence to suggest that Aristotle demanded unity of place. The fact is, Aristotle's formulas were all drawn from Greek tragedy, and these tragedies frequently violated the unities of time and place.