"On Poetics" by Aristotle aims to give an account of poetry. Aristotle does this by attempting to explain poetry through first principles (1447a13), and by classifying poetry into its different genres and component parts. The centerpiece of Aristotle's work is his examination of tragedy. This occurs in Chapter 6 of "On Poetics:"
"Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude; in language embellished with each kind of artistic ornament, the several kinds being found in separate parts of the play; in the form of action, not of narrative; through pity and fear effecting the proper purgation of these emotions." (1449b24-29, SH Butcher transl.)