Whatever ancient theater gossip it provides, the questions “Frogs” raises are ultimately about the canon. (The play’s trio of “great” tragedians — Aeschylus, Sophocles, Euripides — is, after all, the same threesome still taught today.) Who defines it? How do we know what’s “the greatest”? And, most elusively, what are canons for? Do they innocently enshrine “pure” artistic excellence, or is the agenda always somehow political? (“Save the city,” indeed.)

These old questions hover once again on the 20th anniversary of Harold Bloom’s controversial “The Western Canon.” Published at the height of the culture wars, the book ardently defended the idea of works whose aesthetic value was self-evident from what Bloom dismissed as “the school of resentment” — the feminist, deconstructionist and Marxist critics for whom a rigid curriculum of “Great Books” was anathema. To Bloom’s nemeses, the canon was merely a redoubt of white, male, imperialist values: a cultural Stonehenge, as the Oxford literary critic Terry Eagleton put it, best curated by the National Trust.

What’s interesting today is how dated this controversy can seem. One big difference between 1994 and now is the Internet. Bloom and his critics were writing just before the remarkable proliferation of online vehicles for cultural discussion and criticism of books, movies, theater, dance and music, which has radically altered the balance among academic, professional and amateur criticism. Today, audiences as well as critics play a lively role in establishing which works get discussed, analyzed, noticed; the boil of resentment toward the literary gods — the Dionysuses who alone were once privileged to enshrine authors — has been lanced.

From the vantage point of two decades it’s clear, too, that there was justice on both sides. Much depends on whether you see canons as artificial or organic, prescriptive or descriptive. The traditional canon — still prescribed, like intellectual medicine, on many a curriculum — is, in fact, good for you: It provides invaluable insight into the thinking in the past that has helped form the present. (Americans familiar with Enlightenment thinkers like Locke and Rousseau have a deeper understanding of their own polity because those authors profoundly influenced the founders.)

But canons also inevitably shift and expand, not merely enshrining the thought of the past but reflecting an evolving perception of the worlds and cultures we inhabit in the present. This is why a reasonable canon today will include voices that went unheard in the 18th century — or even 50 years ago: the novels of Toni Morrison, with their insistence on acknowledging the moral legacy of American slavery, or the works of Derek Walcott, with their rueful vision of the inheritance of European colonialism.

That the strident posturing of 1994 — the self-dramatizing “elegies” for the death of literature that frame Bloom’s book, his opponents’ preposterous ejection of Proust and Joyce from their syllabuses — now seems passé itself reminds us that cultural landscapes change. As, indeed, do literary reputations. Just ask Aristophanes’ Euripides, who learned to his cost that yesterday’s superstars sometimes end up croaking away in the dark.

Daniel Mendelsohn is the author of seven books, including the international best seller “The Lost: A Search for Six of Six Million”; two collections of essays on books, film, theater and television; and a translation of the poetry of Cavafy. His essays and reviews have appeared frequently in The New Yorker, The New York Review of Books and The New York Times Book Review. Mendelsohn has won the National Book Critics Circle Award for memoir and the NBCC’s Nona Balakian Citation for Excellence in Book Reviewing; the National Jewish Book Award; and the George Jean Nathan Prize for Drama Criticism. His most recent book, “Waiting for the Barbarians: Essays From the Classics to Pop Culture,” was a finalist for the NBCC award in criticism and the PEN Art of the Essay prize. He teaches at Bard College.