If all marriages are “hideously private,” Eliot’s was especially so. Perhaps because she and Lewes were shunned by much of mainstream Victorian society, they carved out what the novelist characterized in letters as a “shared solitude” and “a dual egoism.” As if to dramatize the insularity of their romance, “the letters exchanged between Eliot and Lewes were buried with them in Highgate Cemetery,” Carlisle notes with a twinge of regret.

This archival frustration would present only a minor obstacle if “The Marriage Question” mounted less of an inquiry into Eliot’s marriage and more of an inquiry into the fraught institution in general, as it sometimes purports to do. In the book’s preface, Carlisle laments that “marriage is rarely treated as a philosophical question” and suggests that Eliot’s fictions — and her unorthodox biography — may help her disciples construct a grander theory of intimacy. “The marriage question,” it seems, may just be a matter of thinking more conceptually about love and cohabitation.

In fact, there is no shortage of meditations on marriage, which has been probed from every conceivable angle by everyone from socialist-feminist Emma Goldman to arch-conservative philosopher Roger Scruton. Still, a thinker as humane as Eliot surely has bright light to shed. Unfortunately, “The Marriage Question” does not tell us much about her philosophy of anything, much less her philosophy of love. The book’s broader claims are largely platitudes: It will come as no surprise to anyone, for instance, that marriage “stretches out through time, into the future, growing and changing,” or that “all creatures — including all humans — are shaped by their surroundings.”

Luckily, Carlisle’s latest fares better as a work of partial biography than as a work of philosophy. It passes quickly over Eliot’s birth in 1819 and picks up steam when she is a lovelorn teenager languishing in the British countryside. Her mother died when she was 16, and her sister married early, leaving her to weather a series of stinging romantic rejections alone. First, she developed unreciprocated feelings for her German and Italian tutor; later, she fell head over heels for the scientist (and notorious social Darwinist) Herbert Spencer, again to no avail. Throughout her youth, she was awkward and self-conscious. “At a party,” Carlisle reports, “she stood in a corner, unable to join in the dancing and flirting.” At no point could she forget that she fell “far short of the feminine ideal. Though her figure was slender and graceful, she had a large manly nose, a long chin, ‘evasive’ grey-blue eyes,” and, worst of all, “a formidable intellect and a brooding, sensitive disposition.” As her 21st birthday loomed, she began to grow pessimistic about her prospects. “Every day’s experience seems to deepen the voice of foreboding that has long been telling me, ‘The bliss of reciprocated affection is not allotted to you under any form,’” she wrote.

Fourteen years later, when she boarded a ship for Frankfurt with Lewes, this dire prophecy would prove false. In the meantime, she became a translator from German and Latin, an essayist, and an editor of the Westminster Review, a prestigious progressive journal. Though her intellectual accomplishments probably terrified many of the men in her milieu, they attracted Lewes, who was “well-known on the literary scene as a talented, prolific writer.” He was “small and slight” and notoriously ugly, but he was formidable: By the time he met Eliot, he had published “a book on the French philosopher Auguste Comte, a four-volume ‘Biographical History of Philosophy,’ and numerous reviews and articles — on Goethe, on Spinoza, on Hegel.” (Later, he would turn his capable mind to science writing.) He was ideal in all respects but one: He was not legally separated from his first wife, Agnes, who was publicly and quite placidly conducting an affair with a friend of his, the journalist Thornton Hunt.

Eliot and Lewes were not dissuaded, and scandal ensued. Eliot’s brother stopped talking to her, and when she returned to England, she found she was no longer invited to parties or dinners. But her banishment was worth it, for interpersonally, bliss prevailed. When Eliot describes the routine the couple would follow almost without interruption for the next 25 years in one of her letters from Germany, we can almost hear her beaming: “We work hard in the mornings till our heads are hot, then walk out, dine at three and, if we don’t go out, read diligently aloud in the evening. I think it is impossible for two human beings to be more happy in each other.” Soon, she was calling herself “Mrs. Lewes.”

It was Mr. Lewes who first encouraged his gifted wife to try her hand at fiction. In the 1850s, Mrs. Lewes adopted the pseudonym George Eliot, and several years later, she was a nationwide sensation, beloved even by Queen Victoria. As her wealth and fame burgeoned, Lewes served as her agent, therapist, confidant, editor and biggest acolyte. Knowing that she was paralyzingly sensitive to censure, he encouraged her longtime editor to soften his criticisms and clipped reviews of her work out of the periodicals she liked to read. Everyone agreed that Lewes was doting, even reverent. One visitor to the pair’s household remarked that Eliot “was his chief topic of conversation, the pride and joy of his life.” In a delightful essay about the duo, the literary critic Elizabeth Hardwick wrote that Eliot and Lewes were “inconceivable as anything except what they were, two writers, brilliant and utterly literary. They led the literary life from morning to midnight, working, reading, correcting proofs, traveling, entertaining, receiving and writing letters, planning literary projects, worrying, doubting their powers, experiencing a delicious hypochondria.” What could be better?

Carlisle sometimes casts doubt on this charming picture, suggesting that Lewes may also have been domineering and controlling. After all, the great author’s “earnings were still sent to Lewes, and deposited in his bank account.” But she refrains from definite accusations and even definite pronouncements. For all of Carlisle’s diligent research about Eliot’s context and intellectual genealogy — the author makes forays into the enduring influence of Romanticism, offers lucid expositions of Spinoza — she makes no attempt to answer “the marriage question,” or even to pose it precisely.

Murdoch cautioned that anyone who pries into a marriage will be punished by “an avenging deity.” Perhaps the avenger in this case is the chaotically and cacophonously hybrid genre that is increasingly prevalent in the catalogues of major publishers. “The Marriage Question” is not quite biography: There are several comprehensive biographies of Eliot already. But it is not quite criticism, either: When Carlisle does touch on particular works, she often produces clichés (Eliot’s books “open our eyes and stretch our souls”). Of course, there are some anomalously good books in this daringly dissonant mode — Daniel Mendelsohn’s “Three Rings” is a rare coup that successfully triples as memoir, criticism and history — but casualties of its wide-ranging demands are far more common. “The Marriage Question” tries to do too much and ends up doing little. It is full of airy rhetorical queries: “Why are we so intensely curious about other people’s relationships?” “How could marriage be a site for philosophy, even a path towards knowledge?”

As for Eliot and Lewes’s “marriage,” Eliot aficionados will learn little that they do not know already. That keenly private treasure remains safely behind the curtain after all.

Becca Rothfeld is the nonfiction book critic for The Washington Post.

George Eliot’s Double Life

Farrar, Straus & Giroux. 369 pp. $30