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 critics, 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: Nor, if such another poet could arise, should I very vehemently reproach him, that his first act passed at Venice, and his next in Cyprus. Such violations of rules merely positive, become the comprehensive genius of Shakespeare.
In the 59th paragraph of his famous "Preface" (1765) to his edition of Shakespeare's plays Dr. Samuel Johnson, a famous neo-classical critic was liberal and judicious in applying the rigid neo-classical doctrines to judge Shakespeare's genius. He does so in the following manner:
1. "impossible to decide, and useless to inquire." Johnson begins his defense by remarking that we must first of all give Shakespeare the benefit of doubt as to whether he was aware or unaware of the 'three unities;' and if he had been aware whether he neglected to observe them deliberately. Johnson says that this matter cannot be established with absolute certainty and so he excuses Shakespeare's failure to observe the three unities as "happy ignorance."
2. His success as a playwright: Johnson defends Shakespeare by remarking that Shakespeare's rising fame obviated the need for him to observe 'the three unities':
when he rose to notice, he did not want the counsels and admonitions of scholars and critics
Johnson 'reasonably supposes' that Shakespeare might have accidentally failed to follow 'the three unities' and since the result of his failure to do so only made him more famous he would have continued not observing them.
3. The unity of action: Johnson remarks that only the 'unity of action' is the vital principle which Aristotle had emphasized in his "Poetics" and that the other two unities of time and place "arise evidently from false assumptions." Johnson defends Shakespeare by implying that since Shakespeare has observed the vital principle of "unity of action" his failure to observe the other two unities of 'time' and 'place' can be excused.
4. The uniqueness of Shakespeare's genius: Johnson defends Shakespeare's neglect of 'the three unities' by remarking that Shakespeare's "comprehensive genius" converts the "violations" of the 'three unities' into positive aesthetic results. Johnson attests to the uniqueness of Shakespeare's genius by agreeing not to fault another poet - "if such another poet could arise" - if he could produce the same aesthetically pleasing effects which Shakespeare produced by violating 'the three unities.'
Dr. Johnson defends Shakespeare's "neglect of the unities" in quite a few ways. It is an aesthetic defense, definitely, whereby Johnson shows us that the deviations from this classical norm in Shakespeare are always driven by dramatic necessity. Moreover, though Shakespeare does take liberties, he does not flout the basic realism of the work i.e. he does not degenerate into the ludicrous and the unbelievable.
Johnson also questions the norm of three unities itself. He rightly observes that Aristotle only talks about the unity of action in Poetics and Shakespeare is surely true to that. The unities of time and place are rather loose derivations from Aristotle's text and are suspect in themselves.