(Jade Schulz/for The Washington Post)

Robert Zaretsky teaches at the University of Houston. His latest book is “Catherine & Diderot: An Empress, A Philosopher and the Fate of the Enlightenment.”

As I prepare for my 30th year as a humanities professor, I have noticed an odd paradox shared by the American academy and the Democratic Party: While our political tenets are progressive, our professional tendencies are conservative.

Exhibit A: the absence of a mandatory retirement age.

Come Election Day, subtract the combined ages of Bernie Sanders, Joe Biden and Elizabeth Warren from 2020. You’ll wind up at 1793, when the ink was barely dry on the very document our current president is stress-testing: the Bill of Rights.

Now, do the same calculation with any three of my tenured colleagues in the liberal arts. The odds are good, and getting better, that their combined ages will carry you back to a time when, well, the liberal arts counted for something.

A study by TIAA-CREF revealed that the number of professors who are 65 or older nearly doubled between 2000 and 2010. Nothing suggests this trend has slackened. Why would it? Another study, by Fidelity Investments, showed that of those faculty boomers delaying retirement because of professional reasons, 89 percent “want to stay busy and productive,” while 64 percent “love the work too much to give it up.”

But there is a price to pay for their persistence. (Full disclosure: I will turn 65 next year.) First, it is one of the factors that turn their younger colleagues into permanent temps — adjunct professors, condemned to a Sisyphean existence in which they push the boulder of full-course loads every semester, only to repeat the endeavor the following term.

Moreover, it is unclear how “productive” those baby boomers are. There are no studies I could find that measure the scholarly output of academics who are 65 and over. Though there are admirable exceptions, few of my colleagues of that age show much evidence of such activity.

Can we blame them? After all, if a book falls from a library shelf and no one hears (much less reads) it, does it make a sound? The traditional model of scholarly productivity — the publication of monographs and peer-reviewed articles — is aimed largely at fellow academics, who themselves mostly ignore it. At this year’s Modern Language Association convention, a session called “Game the Name: Crafting the title of your book or dissertation” informed attendees that the “title is the only part of [their] book that most scholars will read.”

The tragedy is this: Baby-boomer academics are less and less active even as the few professional activities they pursue are less and less relevant. Senior academics often justify their presence by citing the need to mentor younger generations. But when it comes to younger faculty, this seems to mean little more than advising them how to write and publish books and articles no one reads. As for their students, too often it means — it has, I confess, in my case — repeatedly teaching the same courses on the same books with the same notes. As one of our articles might phrase it, we assert our hegemony by privileging a particular paradigm while eliding others. In plain English, we have a shtick and won’t give it up.

There is another issue, more delicate yet equally important. Recent studies all confirm that key cognitive abilities begin to fray dramatically after the age of 70. Skills such as concept formation, abstraction and mental agility begin to falter, as does response inhibition, defined as the ability to inhibit an automatic response in favor of producing a novel one. In other words, we increasingly become, quite literally, thoughtless — a less-than-desirable quality for a professor.

And so, we keep on keeping on while student enrollment in the liberal arts keeps on keeling over. Over the past decade, the number of history majors has fallen 25 percent, English majors by more than 20 percent and philosophy majors by 15 percent. Certain universities are eliminating entire humanities departments, while an increasing number of small liberal arts schools are folding up. The groves of academe risk turning into a graveyard.

Though this trend might by now be irreversible, we must still answer a few critical questions: How can we profess the causes of history, literature and philosophy when we remain wedded to monographic scholarship and yellowing lecture notes? Where can we find common ground between the 20-somethings whose tuition pays university salaries and the 70-somethings who pretend to teach them? Finally, can we afford to believe that we are as indispensable as we are indestructible?

Clearly, the American Association of University Professors has no more plans than the Democratic National Committee to propose even a voluntary mandatory retirement age. Yet, with the futures of the academy and democracy equally at stake, my generation needs to think as carefully as possible whether the time has come to turn the helm over to the next generation. If wisdom comes with experience, we are surely wise enough to find ways to retire while continuing to share our knowledge.