What is the theory of three unities? How does Shakespeare violate the three unites and how does Johnson defend him?

The theory of three unities was established by Aristotle in his work On the Art of Poetry and held sway in early modern Europe because of the work of classical humanists during the Renaissance.

Aristotle said that any drama needed to feature unity of place (i.e., it should have only one setting). Shakespeare clearly violates this principle in many of his dramas. Macbeth, for example, begins on a battlefield and has scenes in Macbeth's castle and England, among other places. Second, Aristotle thought that a drama should only portray events over the course of a single day (this is unity of time). Many of Shakespeare's plays, Romeo and Juliet, for example, take place over the course of several days or even longer. Finally, unity of action suggests that the plot should not include any digressions or subplots. Some of Shakespeare's plays (Macbeth, for example) could be said to follow this principle, but others, like King Lear, are quite complex, featuring a number of intriguing subplots. 

Johnson acknowledges several of Shakespeare's shortcomings, but defends his decision to violate the unities. As to unity of place, Johnson denies that the audience has to see the changing scenes as believable. Playgoers, he says, "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." Similarly, he argues that audiences are perfectly capable of imagining that a significant amount of time has elapsed between events in Shakespeare's plays. These lapses, in any case, are usually between acts anyway. "In contemplation," he writes, "we easily contract the time of real actions," and "a lapse of years is as easily conceived as a passage of hours." Indeed, Johnson portrays Shakespeare's willingness to shed structural norms as a virtue, because, as he says, the purpose of a great play is "to copy nature and instruct life." In any case, Johnson believes it possible that Shakespeare did not even know about Aristotle's unities when he wrote his plays.