Dr. Johnson was a classicist with a certain degree of flexibility in his critical theory. He wrote no regular critical treatise. His critical observations are found in some of his papers published in the 'Rambler' and 'Rasselas', and in his 'Preface to the plays of Shakespeare' and 'Lives of the Poets'. His critical theory was basically classical but he did not adhere slavishly to the principles laid down by the ancients. To the classical theory he also added his own personal view together with the historical and biographical approach. He himself said that he sought "to determine upon principles the merit of composition. He equated unguided taste with 'Caprice' and considered unprincipled criticism 'haphazard'. He believed that the rules laid down by the ancients must be subject to change, for "every new genius produces some innovation which, when invented and approved, subverts the rules which the practice of foregoing authors had established".
Dr. Johnson said, "The arbitrary edicts of legislators who, out of various means by which the same end may be attained, selected such as happened to occur to their own reflection". Their application cannot be universal. He admitted that some of those principles were "fundamental and indispensable" and must be adhered to. A true critic should "neither violate essential principles by a desire of novelty, nor debar himself from the attainment of beauties within his view by a needless fear of breaking the rules".
According to Johnson, the historical conditions, the background of the age, and the biographical factors must be taken into consideration when evaluating the work of an author. This is what he does in The Lives of the Poets which is his most outstanding biographical and critical work.