The dramatic unities are three : the unity of action, the unity of time, and the unity of place. Ever since the Renaissance two reasons were advanced in support of the three unities. First, that Aristotle had enjoined them, and secondly, that, they are necessary to create dramatic illusion and in this way to make the drama credible and convincing. During the Pseudo-classical era, the unities were made into rigid rules and their observance was considered essential.
In fact, Aristotle, has stressed only one unity, that is, of action. According to him, the action of a tragedy must be a complete whole and must have an organic unity. The plot is like a living organism, and its every part should be related to the other part or parts naturally and harmoniously. Various incidents must bear a proportionate and harmonious relationship. There should be nothing superfluous. It should not be possible to take out any character or incident, without causing any injury to the plot. Digression and episodes may be introduced but then they should be integral to the plot. Aristotle regards 'episodic plots' as the worst, for they are plots in which the episodes have not properly been interlinked with the main design and do not form an organic part of the whole.
It is the unity of action which makes the plot intelligible, coherent, and individual. The events and incidents are connected with each other logically and inevitably on the principle of probability; they move towards a common goal, the Catastrophe, aimed at by the dramatist. The beginning leads logically and inevitably lo the end, without any unnecessary digressions and episodes coming in between, without there being any detached scenes or incidents.
Aristotle's comment on the length of tragedy gave rise to the doctrine or Unity of Time. According to some critics, when Aristotle asserts that tragedy attempts, as far as possible, to remain within one revolution of the sun, he is referring to the time covered by the dramatic action of the play. The neo-classicists believe that the spectators would not believe in the reality of an action that compressed several days or years into a three-hour drama. And if the spectators did not believe in the reality of an action that the tragedy would not have its proper effect. The idea was carried to absurd extremes. The tendency of the twentieth-century critics has been to reject the notion that Aristotle formally advocated Unity of Time in the Poetics. In the first place, not all Greek tragedies confine their action to a single revolution of the sun, in this sense The Agamemnon and Eumenides are well-known examples of plays that cover several days. Secondly,the neo-classic theory of verisimilitude recommending the unity of time is a false theory of 'copying' different from Aristotle's theory of imitation. Thirdly, there have been plays violating the unity of time and yet have been successful in arousing pity and fear.
Aristotle does not even mention the unity of place. He merely says that the epic may narrate several actions taking place simultaneously at several places, but this is not possible in tragedy which does not narrate it enacts the action. This remark led the Renaissance critics to hoist the unity of place on Aristotle, and on the basis of his authority to make it into a rigid rule for dramatic composition. It was said that in drama there should be no change of place, and even if the scene changes it must not be too great a distance. No doubt, the Unity of place was generally observed by the Greek writers of tragedy for several obvious reasons. There were no drop-scenes, and no division into Acts and Scenes, and so naturally the action was continuous and unbroken. But by this should not be meant that Aristotle prescribed the unity of place. The plays of the Elizabethans incorporate scenes of various places and action and their plays moves from one city to another city, from one country to another.
Hence the unity of action is the higher and controlling law of the drama. If the unity of action is maintained, the other two unities will take care of themselves. The unities of time and place are only of a secondary and purely derivative values. In England, Dr. Johnson gave a death blow to the unities, and nothing has been heard of them ever since. Their interest now is merely historical.