Why did Ian McEwan write “The Children Act”? An odd question, maybe. Why does anyone write anything, especially fiction? And yet, unlike the great books, or even just the very entertaining ones, this novel has a certain quality of the school essay or the magazine assignment. It seems as if it were written in response to a prompt, and that makes it very bizarre.

Mr. McEwan is widely considered one of the finest writers of postwar English fiction. I’ve never been in the camp of his admirers; his work is polished too smoothly, even when it is at its grimmest. It’s too imperturbable for my taste.

Nevertheless, no writer can deny that at the level of the sentence and the paragraph, there are few better. Mr. McEwan rarely misplaces a word.

His prose, whether describing a setting or paraphrasing a character’s interior state, has that seemingly impossible combination of balance and momentum. It combines quietude and motion, like a perfect vinaigrette in which the oil and vinegar are beat into a smooth emulsion.

Odd, then, that the first writer who came to mind as I read his latest was not some other novelist of exquisite sentences — André Aciman in “Call Me By Your Name” or Zadie Smith in “On Beauty” — but rather David Brooks. Yes, the newspaper columnist. Mr. Brooks, a milquetoast conservative opinion writer for The New York Times, is hardly graceful, and he has an undue fondness for pop science, especially pop neuroscience.

In 2011, Mr. Brooks wrote a clunky, semi-novelistic, sort-of nonfiction book called “The Social Animal,” in which a pair of fictional characters named Harold and Erica proceeded through a series of life scenarios that bore a remarkable similarity to those of people much like David Brooks — that is to say: rich, successful and Washingtonian. The book is stuffed with the latest bits of neuro-biology, the sort of thing you’d hear in an interesting three-minute interview on NPR.

Why then did “The Children Act” remind me so much of this seemingly unrelated and unsuccessful experiment in nonfiction fiction? Mr. McEwan’s novel is a brief, almost ascetic story of Fiona Maye, a British High Court judge, and her interaction, through a case that comes before her court, with a young man (or boy — the distinction is significant in the proceedings) who suffers from leukemia and refuses, for religious reasons, to submit to a bone marrow transplant. He and his parents are Jehovah’s Witnesses.

Maye must decide if she will overrule the family’s wishes and allow the hospital to administer treatment to the 17-year-old juvenile. Without it he will almost certainly, and horribly, die.

What follows, hemmed in by short digressions into other cases that Maye has heard and judged as well as an ongoing domestic dispute with her husband, who has admitted to desiring an affair, is a delicate disquisition on how the modern, liberal, administrative state as represented by a modern, liberal, exquisitely sensitive and generally judicious person of excellent intellect and education, will navigate the challenge that religious fundamentalism poses to its perception — and enforcement — of social welfare and the general good.

In other words, what does someone like Ian McEwan think about all this stuff? The result is a precarious balancing, first welcome and happy, ultimately rather sad. Mr. McEwan’s characters are generally likable and well-meaning (a departure, actually, from some of his earlier work).

They are frequently competent. But for all this, and for the excellent quality of the prose, in the final judgment, it all reads like someone’s graduate school exams, a clever setting for a moral argument toward a problem artificially contrived to perplex.

Because of this, a late moment of self-betrayal feels vestigial, a hasty caveat inserted to muss the neat arrangements that were settled on before. A younger, less successful, less experienced author would never be permitted to publish such a book, and in one way, it’s a testament to Mr. McEwan that he was willing to attempt it.

The result, unfortunately, neither rewards nor challenges very much. The tragedy is merely wistful, and the moral sentiments mostly banal.

First Published: September 28, 2014, 4:00 a.m.