The Preface to The Plays of William Shakespeare—often referred to as "Preface to Shakespeare" in its stand-alone version—appears in the first volume of the eight-volume edition of Shakespeare's plays edited by Samuel Johnson, and published in London in 1765.
The Preface is divided generally into three parts. In the first part, Johnson extolls Shakespeare's virtues. In the second part, Johnson discusses the twelve faults that he finds in Shakespeare's plays, including Shakespeare's failure to conform to the "three unties" of action, time, and place. In the third part, Johnson expresses his own opinion about the "three unities."
The "three unities"—the unity of action (that the play have a single, unified plot), of time (that the action of the play occurs within a single day), and of place (that the action of the play occurs in a single location)—are attributed to Aristotle in his Poetics, which he wrote around 335 BCE.
Poetics is based on Aristotle's analysis of the plays of Aeschylus, Sophocles, Euripides, and other ancient Greek playwrights, all of whom wrote their plays a hundred years or more before Aristotle wrote Poetics. Aristotle's comments about action, time, and place in tragic plays were observations, and his observations weren't intended to be rules for playwrights to follow.
In Poetics, Aristotle defines a tragic play as "an imitation of an action that is serious, complete, and of a certain magnitude..." (Poetics, VI).
By "complete," Aristotle means that the plot—the "action" of the play—should be self-contained, with a single, unified plot, and no subplots.
The plot, being an imitation of an action, must imitate one action and that a whole, 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" (Poetics, VIII).
In other words, a tragedy has a single, unified plot with a clearly defined beginning, middle, and end, and no subplots.
Aristotle mentions the "unity of time" only as an observation.
Tragedy endeavors, as far as possible, to confine itself to a single revolution of the sun, or but slightly to exceed this limit. (Poetics, V)
Aristotle doesn't mention "unity of place" at all.
It was French neoclassic writers and critics—some of whom were living at the same time as Samuel Johnson, and who were calling Shakespeare to task for violating the "three unities"—who formulated and formalized the "three unities" as inviolable rules for plays of their period.
Johnson believes, as did Aristotle, that a unity of action was all that was necessary in a tragedy, comedy, or history play. Johnson argues that although Shakespeare wrote subplots into his plays, even the subplots contribute directly to the main action and unity of the play. Johnson admits that Shakespeare clearly violated the unities of time and place almost without exception in his plays, but asserts that nothing is essential to Shakespeare's plays except a unity of action, to which Shakespeare steadfastly adheres.
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.
Johnson refutes the neoclassicists' insistence on the unities of action, time, and place as a rejection of what we know and accept as an essential part of the enjoyment of a performance of a play—"a willing suspension of disbelief."
The neoclassicists believed that the audience should view the performance of a play as a representation of reality, or a "resemblance of reality." Johnson believes that the audience's imagination—what he calls "delusion"—should not, and, in fact, cannot be limited arbitrarily by imposing the "three unities" on a play or dramatic presentation.
Delusion, if delusion be admitted, has no certain limitation; if the spectator can be once persuaded, that his old acquaintance are Alexander and Caesar, that a room illuminated with candles is the plain of Pharsalia, or the bank of Granicus, he is in a state of elevation above the reach of reason, or of truth, and from the heights of empyrean poetry, may despise the circumscriptions of terrestrial nature. There is no reason why a mind thus wandering in extasy should count the clock, or why an hour should not be a century in that calenture of the brains that can make the stage a field.
Johnson succinctly summarizes his argument against the imposition of the "three unities" as absolute requirements of playwriting.
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.