When we are young we busy ourselves in forming schemes for succeeding time, and miss the gratifications that are before us; when we are old we amuse the languor of age with the recollection of youthful pleasures or performances; so that our life, of which no part is filled with the business of the present time, resembles our dreams after dinner, when the events of the morning are mingled with the designs of the evening.

Literary criticism in Johnson's time, as in ours, was dominated by French theory. Johnson's riposte is English common sense. Where the French tangled themselves in the rules of art, Johnson's only principle was truth to life. Voltaire threw up his hands in horreur at Shakespeare's mingling of tragedy and comedy, kings and clowns. Johnson replies that that is how life is:

Shakespeare's plays are not in the rigorous and critical sense either tragedies or comedies, but compositions of a distinct kind; exhibiting the real state of sublunary nature, which partakes of good and evil, joy and sorrow, mingled with endless variety of proportion and innumerable modes of combination...in which, at the same time, the reveler is hasting to his wine, and the mourner
burying his friend.

Johnson's own public persona was deeply Shakespearean, one part moralist, the other part stand-up comedian. A Tory in an age of Whig supremacy, he was anything but politically correct. His notorious prejudices were above all provocations to debate. He loved to bait Boswell, his Scotch biographer. Hence the great riff on "noble prospects" of the Scottish landscape:

"I believe, Sir, you have a great many. Norway, too, has noble wild prospects; and Lapland is remarkable for prodigious noble wild prospects. But, Sir, let me tell you, the noblest prospect which a Scotsman ever sees, is the high road that leads him to England!" 

Boswell knew how to take it on the chin: he records that "this unexpected and pointed sally produced a roar of applause." It's all in the comic timing.