The unities of action (a single plot), of time (the action of the play occurs within a single day), and of place (the action of the play occurs in a single location), although attributed to Aristotle (384–322 BCE), were actually formalized by Lodovico Castelvetro (1505–1571 CE) and other French classicists in the late 1500s.
Aristotle wrote Poetics around 335 BCE, a hundred years or more after the plays of Aeschylus, Sophocles, Euripides, and other Ancient Greek playwrights were written and performed. The three unities ascribed to Aristotle were actually based on Aristotle's observation and analysis of Ancient Greek plays and were not rules that Aristotle formulated for playwrights to follow.
[T]he plot manifestly ought, as in a tragedy, to be constructed on dramatic principles. It should have for its subject a single action, whole and complete, with a beginning, a middle, and an end." (Poetics, part 23)
"A single action" means that the play should have one plot, no subplots, and there should...
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