Any discussion of the "three unities" in Oedipus Rex invariably includes reference to Aristotle's Poetics as the source of those unities.
In Poetics, Aristotle emphasizes only one unity—the "unity of action"—as absolutely essential to a tragedy.
Aristotle defines a tragedy as " an imitation of an action that is serious, complete, and of a certain magnitude" (Poetics, VI).
The important word regarding the "unity of action" is "complete," meaning that the plot should be self-contained, with a beginning, middle, and end, and that:
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 (Poetics, VIII).
It [a tragedy] should have for its subject a single action, whole and complete, with a beginning, a middle, and an end. It will thus resemble a living organism in all its unity (Poetics, XXIII)
In other words, a tragedy has a single,...
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