In the "Preface" to the eight-volume edition of The Plays of William Shakespeare edited by Samuel Johnson (1709–1784) and George Steevens (1736–1800), published in London in 1765, Johnson argues against critics who find fault with Shakespeare's plays for his failure to conform to the French neoclassical ideals of the "three unities" of action, time, place, and other arbitrary rules imposed on plays and playwrights.
In 1635, Cardinal de Richelieu was granted permission by King Louis XIII to institute the Académie Française (the French Academy), whose forty members-for-life were charged with the task of standardizing, purifying, and preserving the French language.
Richelieu expanded the role of the French Academy to include establishing guidelines and rules for the arts, including theatre, based on Neoclassical ideals.
The French Academy determined that plays must conform to these rules:
Decorum. Characters must act in a morally acceptable manner that's appropriate to their place in society.
Verisimilitude. The action of a play must be representative of everyday life, with no supernatural or fantastical elements.
Unity of Time. The action of the play must occur in a time span of no more than twenty-four hours.
Unity of Place. The action of the play must occur in one location, later amended to include any location that could be reached in the twenty-four time span of the play.
Unity of Action. No more than one plot, and with as few characters as possible.
Genre. Elements of tragedy and and comedy should never be mixed in the same play, and the purpose of every play was to teach a lesson.
The French Academy also imposed rules specific to tragedy and comedy.
Shakespeare's plays break all of these rules—sometimes within the same play—but Johnson asserts in the "Preface" that none of these rules should apply to Shakespeare's plays, or any other plays, except for the unity of action.
Johnson admits that Shakespeare broke that rule, too, but he argues that the subplots in Shakespeare's plays contribute directly to the main plot and the overall action of the play:
His [Shakespeare's] histories, being neither tragedies nor comedies, are not subject to any of their laws; nothing more is necessary to all the praise which they expect, than that the changes of action be so prepared as to be understood, that the incidents be various and affecting, and the characters consistent, natural and distinct. No other unity is intended, and therefore none is to be sought.
In his other works he has well enough preserved the unity of action...His plan has commonly what Aristotle requires, a beginning, a middle, and an end; one event is concatenated with another, and the conclusion follows by easy consequence. There are perhaps some incidents that might be spared, as in other poets there is much talk that only fills up time upon the stage; but the general system makes gradual advances, and the end of the play is the end of expectation.
It's interesting to note that the ancient Greek tragedies of Aristophanes, Sophocles, Euripides, and others which Aristotle analyzed with regard to the unites of action and time that he explored in his Poetics—there's no mention of the unity of place in Poetics—are almost exclusively historical plays. The plays tell the well-known stories of mythical and historical figures in Greece's past.
Following Johnson's argument, even the ancient Greek plays from which the unities are derived shouldn't be subject to the rules of the French Academy.
These idealized Greek plays on which most of the French Academy's rules are based should also be criticized for violations of verisimilitude for the inclusion of supernatural beings—notably the appearance of gods in the deus ex machina scenes.
Johnson makes another argument from the point of view of the audience, which he considers of far greater importance than the patronizing view of the French Academy or any critic:
The truth is, that the spectators are always in their senses, and know, from the first act to the last, that the stage is only a stage, and that the players are only players. They come to hear a certain number of lines recited with just gesture and elegant modulation. The lines relate to some action, and an action must be in some place; but the different actions that compleat a story may be in places very remote from each other; and where is the absurdity of allowing that space to represent first Athens, and then Sicily, which was always known to be neither Sicily nor Athens, but a modern theatre?
Although Johnson calls Shakespeare to task for what he considers minor deficiencies in his plays, he nevertheless holds Shakespeare in high regard as a playwright, and dismisses any criticism of his plays based on the French Academy's rules as wholly irrelevant:
Whether Shakespeare knew the unities, and rejected them by design, or deviated from them by happy ignorance, it is, I think, impossible to decide, and useless to inquire. We may reasonably suppose, that, when he rose to notice, he did not want the counsels and admonitions of scholars and criticks, and that he at last deliberately persisted in a practice, which he might have begun by chance. As nothing is essential to the fable, but unity of action, and as the unities of time and place arise evidently from false assumptions, and, by circumscribing the extent of the drama, lessen its variety, I cannot think it much to be lamented, that they were not known by him, or not observed.