a) Unity of Action,
b) Unity of Time, and,
c) Unity of Place.
Unity of Action
Unity of actions means only those actions, and not all, in the life of the hero which are intimately connected with one another and appear together as one whole. Aristotle says that '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. There may be many more actions in the life of the hero --there are in every man's life--but unless they have something to do with the tragedy that befalls him, they are not relevant to the plot and will all have to be kept out. FOR A THING WHOSE PRESENCE OR ABSENCE MAKES NO VISIBLE DIFFERENCE, IS NOT AN ORGANIC PART OF THE WHOLE.
Unity of Time
Unity of Time is the conformity between the time taken by the events of the play and that taken in their representation on the stage. "Tragedy," he says, "endeavours, as far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit; whereas the epic action has no limits of time.' By this Aristotle doesn't mean to confine the whole play into an event or events that happened within twenty four hours or so in life. He merely states the prevailing practice but is not unaware of the fact that, in this particular matter, 'at first the same freedom was admitted in tragedy as in epic poetry.'
Unity of Place
Unity of Place is the conformity between the scene of the tragic event or events and the time taken by them to happen.
For a good tragic plot to arouse the emotions of pity and fear in the spectator or reader. Pity, as has been said, happens for the undeserved sufferings of the hero, and fear of the worst that may happen to him. The change of fortune therfore should not be from bad to good, but reversely, from good to bad. A happy ending may please us more but it will not afford the true tragic pleasure-- that aroused by the emotions of pity and fear.
The plot, finally, is divisible into two parts complication, and its unraveling or denouement. The former ties the event into a tangled knot, the latter unties it. The first is commonly called rising action and the second falling action.
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