Unity of plot

Aristotle makes a single precise correction in this brief chapter: unity of plot does not consist in the unity of the hero. A single person's life contains infinitely various incidents that cannot be reduced to a single action. The mistake — making a play or epic about a hero's entire life on the assumption that his unity as a person guarantees the unity of the work — is common and wrong. Aristotle implies he has seen poets make it.

Homer understood this. The Iliad tells not the whole Trojan War, not the whole life of Achilles, but one action within that war — the wrath of Achilles — and its consequences. Even the subsidiary episodes are made necessary or probable in relation to that central action; they are not included merely because they happened to Achilles. The Odyssey works the same way. This is what makes Homer's epics structurally unified despite their length: every part belongs to the whole by necessity or probability, not by the biographical accident of involving the same person. A plot is unified when removing or rearranging any part damages or displaces the whole.