In his Poetics, Aristotle defines tragedy as “an imitation (mimēsis) of an action that is serious, complete, and of a certain magnitude.” He asserts that any tragedy can be divided into six constituent parts: plot, character, thought, diction, song and spectacle, and—the most important of all—the first principle, the plot, described as the “soul of a tragedy.” Furthermore, he states that the key components here are the unity and the wholeness of the plot. Everything that happens, every element and every event, must work together in order to form a whole.
So the plot, being an imitation of an action, must imitate one action and that a whole, the structural union of the parts being such that, if any one of them is displaced or removed, the whole will be disjointed and disturbed.
The plot must have a beginning, a middle, and an end. He argues that a well-structured plot must retain “a length easily embraced by the memory.” If done right, the reader or the watcher will feel a “tragic pleasure of pity and fear” when processing the plot, and, essentially, this is what distinguishes tragedy from other literary genres. The audience usually feels pity for the tragic hero, who, due to known or unknown circumstances, has been through something traumatic and life-changing. The hero must be neither good nor evil, but rather human and real so that the audience can identify with their character, and when they finally face their downfall due to some mistake or lapse in judgement, the audience can empathize. He writes:
Tragedy is a form of drama exciting the emotions of pity and fear. Its action should be single and complete, presenting a reversal of fortune, involving persons renowned and of superior attainments, and it should be written in poetry embellished with every kind of artistic expression.