If one is wishing to say that a certain deceased composer is the best among all, living or dead, ought one to say "Mahler is the greatest" or "Mahler was the greatest"? If the former, can this tense also be used for conductors, for example if one were to say "Bernstein is the greatest"?
It seems possible to me that one might be able to say that "Mahler is the greatest" owing to "Mahler" having become shorthand for "Mahler's works"; again, if so, would this also be true for conductors?
In assessing the greatness of a no-longer-living person, it is certainly possible and reasonable to frame the assessment as one looking into the past, even though you think the person's artistic output remains great today. Thus, you might say
George Eliot was a brilliantly inquisitive and empathetic novelist, not just in her universally acknowledged masterpiece Middlemarch, but in less appreciated works such as Romola, Daniel Deronda, and Felix Holt, the Radical.
Alternatively, you could bring the same assertion into the present, without any alteration other than the verb, and be completely justified:
George Eliot is a brilliantly inquisitive and empathetic novelist, not just in her universally acknowledged masterpiece Middlemarch, but in less appreciated works such as Romola, Daniel Deronda, and Felix Holt, the Radical.
In this case, the past/present issue is just a kind of vantage point that the speaker or writer adopts in discussing the subject. The judgments about Eliot's work take place in a setting not bound by the chronological limits of her own milieu. Consequently, the speaker/writer should feel free to adopt whichever form seems more natural and appropriate in making the particular appraisal at hand.
In my view, using present tense when assessing a historical subject's greatness (or mediocrity) can be valid even outside the special preserve inhabited by authors, painters, composers, and other fine artists of the past. Consider this excerpt from Montaigne's essay "Of the Most Excellent Men," in which the author compares the Theban general Epaminondas with Alexander the Great and Julius Caesar—and Epaminondas wins:
The third and in my opinion the most excellent, is Epaminondas. Of glory he has not near so much as the other two (which, for that matter, is but a part of the substance of the thing): of valour and resolution, not of that sort which is pushed on by ambition, but of that which wisdom and reason can plant in a regular soul, he had all that could be imagined. Of this virtue of his, he has, in my idea, given as ample proof as Alexander himself or Caesar: for although his warlike exploits were neither so frequent nor so full, they were yet, if duly considered in all their circumstances, as important, as bravely fought, and carried with them as manifest testimony of valour and military conduct, as those of any whatever. The Greeks have done him the honour, without contradiction, to pronounce him the greatest man of their nation; and to be the first of Greece, is easily to be the first of the world. As to his knowledge, we have this ancient judgment of him, “That never any man knew so much, and spake so little as he”;—[Plutarch, On the Demon of Socrates, c. 23.]—for he was of the Pythagorean sect; but when he did speak, never any man spake better; an excellent orator, and of powerful persuasion. But as to his manners and conscience, he infinitely surpassed all men who ever undertook the management of affairs; for in this one thing, which ought chiefly to be considered, which alone truly denotes us for what we are, and which alone I make counterbalance all the rest put together, he comes not short of any philosopher whatever, not even of Socrates himself. Innocence, in this man, is a quality peculiar, sovereign, constant, uniform, incorruptible, compared with which, it appears in Alexander subject to something else subaltern, uncertain, variable, effeminate, and fortuitous.
In speaking of Eapaminondas's qualities and essence, Montaigne sets his subject in the present tense, not because these things are artifacts of some kind (like a Mahler symphony or an Eliot novel) but because they are alive to Montaigne's mind.
When the description shifts to a historical detail (such as Epaminondas's membership in the Pythagorean sect or even his warlike exploits), Montaigne easily shifts to past tense. But when the subject is greatness, preeminence, manners and conscience, or innocence, Montaigne presents his man in present tense—because to Montaigne those qualities in that context are outside time.
The only inconsistency I see in Montaigne's discussion of his subject is that he puts Epaminondas's valor and resolution in past tense, as if those were relatively temporal categories of things. But perhaps Montaigne views those qualities—admirable though they may be—as less remarkable and fascinating than the qualities that he brings into the present for consideration.
Anyway, there is no reason to suppose that saying "Mahler is the greatest" or "Bernstein is the greatest" is chronologically incorrect. As a writer or speaker, you get to decide what to leave locked in time and what to free from the past.
Use "is" to make a claim of comparison at the present time. Use "was" if Mahler was the greatest when alive, but now some other composer (say, Vanilla Ice :-) ) has surpassed him.
