Dr. Johnson asks this question in his preface to Shakespeare (1765):
That praises are without reason lavished on the dead, and that the honours due only to excellence are paid to antiquity, is a complaint likely to be always continued by those, who, being able to add nothing to truth, hope for eminence from the heresies of paradox; or those, who, being forced by disappointment upon consolatory expedients, are willing to hope from posterity what the present age refuses, and flatter themselves that the regard which is yet denied by envy, will be at last bestowed by time.
Antiquity, like every other quality that attracts the notice of mankind, has undoubtedly votaries that reverence it, not from reason, but from prejudice. Some seem to admire indiscriminately whatever has been long preserved, without considering that time has sometimes co-operated with chance; all perhaps are more willing to honour past than present excellence; and the mind contemplates genius through the shades of age, as the eye surveys the sun through artificial opacity.
The great contention of criticism is to find the faults of the moderns, and the beauties of the ancients. While an author is yet living we estimate his powers by his worst performance, and when he is dead, we rate them by his best.
To works, however, of which the excellence is not absolute and definite, but gradual and comparative; to works not raised upon principles demonstrative and scientific, but appealing wholly to observation and experience, no other test can he applied than length of duration and continuance of esteem. What mankind have long possessed they have often examined and compared; and if they persist to value the possession, it is because frequent comparisons have confirmed opinion in its favour.
As among the works of nature no man can properly call a river deep, or a mountain high, without the knowledge of many mountains, and many rivers; so in the productions of genius, nothing can be styled excellent till it has been compared with other works of the same kind. Demonstration immediately displays its power, and has nothing to hope or fear from the flux of years; but works tentative and experimental must be estimated by their proportion to the general and collective ability of man, as it is discovered in a long succession of endeavours.
Of the first building that was raised, it might be with certainty determined that it was round or square; but whether it was spacious or lofty must have been referred to time. The Pythagorean scale of numbers was at once discovered to be perfect; but the poems of Homer we yet know not to transcend the common limits of human intelligence, but by remarking, that nation after nation, and century after century, has been able to do little more than transpose his incidents, new-name his characters, and paraphrase his sentiments.
The reverence due to writings that have long subsisted arises therefore not from any credulous confidence in the superior wisdom of past ages, or gloomy persuasion of the degeneracy of mankind, but is the consequence of acknowledged and indubitable positions, that what has been longest known has been most considered, and what is most considered is best understood.
The Poet, of whose works I have undertaken the revision, may now begin to assume the dignity of an ancient, and claim the privilege of established fame and prescriptive veneration. He has long outlived his century, the term commonly fixed as the test of literary merit. Whatever advantages he might once derive from personal allusions, local customs, or temporary opinions, have for many years been lost; and every topic of merriment, or motive of sorrow, which the modes of artificial life afforded him, now only obscure the scenes which they once illuminated.
The effects of favour and competition are at an end; the tradition of his friendships and his enemies has perished; his works support no opinion with arguments, nor supply any faction with invectives; they can neither indulge vanity nor gratify malignity; but are read without any other reason than the desire of pleasure, and are therefore praised only as pleasure is obtained; yet, thus unassisted by interest or passion, they have past through variations of taste and changes of manners, and, as they devolved from one generation to another, have received new honours at every transmission.
But because human judgment, though it be gradually gaining upon certainty, never becomes infallible; and approbation, though long continued, may yet be only the approbation of prejudice or fashion; it is proper to inquire, by what peculiarities of excellence Shakespeare has gained and kept the favour of his countrymen.
The answer, of course, is time, the test of time, but I have never heard anyone put it quite so nicely.