THERE is little need today to speak of "The Three Unities," which once were the panoply of war of critics. They were flung into the heap of things, by Corneille about 1660, although they were drawn from Aristotle and the Grecian stage. The only Unity Aristotle insisted upon was the Unity of Action, throughout a Tragedy: though he did say that Tragedy, as much as may be, tries to keep its action within one revolution of the sun, or nearly so, which gave a formulation of the "Unity of Time." The "Unity of Place" was apparently Corneille's, for he himself allowed that he found it neither in Aristotle nor in Horace. It found its basis, however, quite naturally in the necessity of the construction of the Greek Stage.
The only Unity that has any reason in it, and that really is observed at all in all dramatic works, is the "Unity of Action."  This means, of course, that the interest of the play shall be one throughout. It may have more events than one, but the interest must not be divided; and any subsidiary motive must be purely secondary, lying into the main purpose, and leading towards the general conclusion. In the ancient Tragedy there was no counterpoint. The poet avoided all possibility of divided interest, yet he sometimes had a subsidiary interest beneath the main line of purpose. For example, in "The Persians" there is undivided interest, up to the lamentations of the Persian Councillors, over the national disaster at Salamis; and a new potentiality emerges in the rising of the ghost of Darius, out of the sleep of the shades, stirred by the woe of his country--a true tragic episode. But the interest does not divide itself: it rather passes over into and continues the emotion, deepening it to the close. So also in the "Ajax," the death of the hero is not the end, the close of national interest. The burial of the hero is important, for the pride of the Athenians, who have taken him as their national patron, is therein involved.
In the Elizabethan Drama, especially in Shakespeare, you have a strong unity in many parts, many tides seeking one strait, making the tragic wave which finds resolution at last upon one shore. There may be great complexity of thought and passion, with unity of action. "Macbeth" has the murder of Lady Macduff and her children intensifying the surer under-tragedy of retribution; while the murder of Banquo deepens the interest, and darkens the certainty of judgment; yet the one great stream flows on to the door of doom.
The tragic event does not at once subside into stillness. It would be the most untrue imitation of tragic life were it so represented on the stage.
"Unity of Time" even in the Attic Stage, was not observed. Not a day or a month, no limit of years, can be laid upon the tragic movement.
Sir Philip Sidney's famous objection to the movement of the stage of his time is worthy of reproduction:
"You shall have Asia of the one side, and Affrick of the other, and so many other under-kingdoms, that the Player, when he cometh in, must ever begin with telling where he is: or els, the tale will not be conceived. Now ye shall have three Ladies, walke to gather flowers, and then we must beleeve the stage to be a Garden. By and by, we heare news of shipwracke in the same place, and then wee are to blame, if we accept it not for a rock. Upon the backe of that comes a hideous Monster, with fire and smoke, and then the miserable beholders are bounde to take it for a Cave. While in the meantime, two Armies flye in, represented with foure swords and bucklers, and then what harde heart will not receive it for a pitched fielde? Now, of time they are much more liberall, for ordinary it is that two young princes fall in love. After many traverces, she is got with childe, delivered of a faire boy, he is lost, groweth a man, falls in love, and is ready to get another childe, and all this in two hours' space: while how absurd it is in sence, even sence may imagine, and Arte hath taught and all auncient examples justified."
But it could not stem the movement of the Drama towards the truer representation of human life and its passions. Nor would it have been for the good of Drama had it succeeded; for it would have shut in Tragedy to episodes, and not to a whole action with all that it involved.