'Out of these two [Aristotle and Horace] has been extracted the famous rules which the French call des trois unités, or the Three Unities, which ought to be observed in every regular play: namely, of time, place, and action.
'The unity of time they comprehend in twenty-four hours, the compass of a natural day, or as near as it can be contrived; and the reason of it is obvious to every one, that the time of the feigned action, or fable of the play; should be proportioned as near as can be to the duration of that time in which it is represented; since, therefore, all plays are acted on the theatre in a space of time much within the compass of twenty-four hours, that play is to be thought the nearest imitation of nature whose plot or action is confined within that time; and, by the same rule which concludes this general proportion of time, it follows that all the parts of it are to be equally subdivided; as namely, that one act take not up the supposed time of half a day, which is out of proportion to the rest; since the other four are then to be straitened within the compass of the remaining half: for it is unnatural that one act, which being spoke or written is not longer than the rest, should be supposed longer by the audience; 'tis therefore the poet's duty to take care that no act should be imagined to exceed the time in which it is represented on the stage; and that the intervals and inequalities of time be supposed to fall out between the acts.
'This rule of time, how well it has been observed by the Ancients, most of their plays will witness; you see them in their tragedies (wherein to follow this rule is certainly most difficult) from the very beginning of their plays, falling close into that part of the story which they intend for the action or principal object of it, leaving the former part to be delivered by narration: so that they set the audience, as it were, at the post where the race is to be concluded; and, saving them the tedious expectation of seeing the poet set out and ride the beginning of the course, you behold him not till he is in sight of the goal, and just upon you.
'For the second unity, which is that of place, the Ancients meant by it that the scene ought to be continued through the play, in the same place where it was laid in the beginning: for the stage on which it is represented being but one and the same place, it is unnatural to conceive it many, and those far distant from one another. I will not deny but, by the variation of painted scenes, the fancy (which in these cases will contribute to its own deceit) may sometimes imagine it several places, with some appearance of probability; yet it still carries the greater likelihood of truth if those places be supposed so near each other, as in the same town or city; which may all be comprehended under the larger denomination of one place; for a greater distance will bear no proportion to the shortness of time which is allotted in the acting, to pass from one of them to another; for the observation of this, next to the Ancients, the French are to be most commended. They tie themselves so strictly to the unity of place that you never see in any of their plays a scene changed in the middle of an act: if the act begins in a garden, a street, or chamber, 'tis ended in the same place; and that you may know it to be the same, the stage is so supplied with persons that it is never empty all the time: he that enters the second has business with him who was on before; and before the second quits the stage, a third appears who has business with him. This Corneille calls la liaison des scènes, the continuity or joining of the scenes; and 'tis a good mark of a well contrived play when all the persons are known to each other, and every one of them has some affairs with all the rest.
'As for the third unity, which is that of action, the Ancients meant no other by it than what the logicians do by their finis, the end or scope of any action; that which is the first in intention, and last in execution: now the poet is to aim at one great and complete action, to the carrying on of which all things in his play, even the very obstacles, are to be subservient; and the reason of this is as evident as any of the former.
'For two actions, equally laboured and driven on by the writer, would destroy the unity of the poem; it would be no longer one play, but two; not but that there may be many actions in a play, as Ben Jonson has observed in his Discoveries; but they must be all subservient to the great one, which our language happily expresses in the name of under-plots: such as in Terence's Eunuch is the difference and reconcilement of Thais and Phædria, which is not the chief business of the play, but promotes the marriage of Chærea and Chremes's sister, principally intended by the poet. There ought to be but one action, says Corneille, that is, one complete action which leaves the mind of the audience in a full repose; but this cannot be brought to pass but by many other imperfect ones which conduce to it, and hold the audience in a delightful suspense of what will be.
'If by these rules (to omit many other drawn from the precepts and practice of the Ancients) we should judge our modern plays, 'tis probable that few of them would endure the trial: that which should be the business of a day, takes up in some of them an age; instead of one action, they are the epitomes of a man's life; and for one spot of ground (which the stage should represent) we are sometimes in more countries than the map can show us.