The Three Unities
Aristotle’s theory of the unities of action, time and place, reconstructed from the Poetics, had particular influence in Renaissance Italy and France. In his discussion of plot, Aristotle emphasizes the importance of wholeness in order to achieve the requisite degree of order, amplitude and unity.
Tragedy endeavours as much as possible to confine itself to one revolution of the sun … whereas the Epic action has no limits of time.
A plot should involve the mimesis (or representation) of a whole action, with a beginning, a middle and an end.
The extent to which Aristotle intended these remarks as descriptive observations or prescriptive rules is open to dispute, although he does offer more explicit guidance to practising playwrights.