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

 Aristotle's theory of the three unities states that a play should embrace unities of action, time and place. Unity of action means that a play should follow one plot, without meandering off into subplots. Unity of time means that a play should take place within a 24-hour period, and unity of place means that a play should take place in one spot, not move from place to place. Shakespeare repeatedly violated the three unities by including subplots in his plays, by writing plays that spanned more than 24 hours and by having scenes in his plays take place in different spots, such as, Johnson notes, moving from Cyrpus to Venice in the same play. 

Johnson defends Shakespeare by saying that he does follow unity of action by writing plays that have a clear beginning, middle and end. Johnson then said that, as for time and place, people know they are watching a play and can easily adjust to different times and places. Once people engage imaginatively, they can continue to do so. As Johnson put it,  “He that can take the stage at one time for the palace of the Ptolemies, may take it in half an hour for the promontory of Actium. Delusion, if delusion be admitted, has no certain limitation.” 

Johnson also defends Shakespeare by stating that what determines whether we "believe in" a play is not so much adherence to unities or to the "facts" of history but the emotional integrity of the play. In other words, what is important is how relatable or relevant the play is to our own lives: “The reflection that strikes the heart is not, that the evils before us are real evils, but that they are evils to which we ourselves may be exposed.” Johnson also defends the plays as "pleasurable" and says that their pleasure outweighs any violations of unities.