Explain the concept of tragic unity in Aristotle's Poetics.

Aristotle characterises tragedy as possessed of a unity of
action, time and place. Aristotle described unity of action as follows: “tragedy is an
imitation of an action that is complete, and whole, and of a certain magnitude.” In
other words, the main action of tragedy should be something serious (murder, ritual
pollution, the rise and fall of a ruler) or non-trivial. The tragedy, although it may
start in medias res, and report its backstory, as it were, should conclude with a
resolution. This meant that a tragedy should have a single major plot, in which a
protagonist underwent a reversal of fortune. Unlike epic, the scope of tragedy should be
restricted to a single plot line. The action of a tragedy should take place in a single
24 hour period (although earlier actions may be reported) and should be set in a single
place.