Do you have money? Do you have two thousand employees on the payroll? Are you a banker or a manufacturer or an industrialist? If so, Stendhal doesn’t want you to read Love (De L’amour, 1822); you wouldn’t understand it. But if you are a dreamer, if you are tender, if Mozart moves you, if you nod solemnly when asked, “Have six months of your life ever been made miserable by love?,” this book is for you.

So Stendhal introduces his wonderfully eccentric study of love, framing it as an underground classic by design. “I am writing,” he says, “for a mere hundred readers, unfortunate, likeable, charming, unhypocritical, unself-righteous people whom I wish to please; I know no more than one or two.” He wasn’t far wrong. Stendhal reported only 17 readers between 1822 and 1833. His publisher quipped, “You might call it sacred, for nobody will touch it!”

Although Love remains relatively unknown, Marcel Proust, Simone Weil, and W. G. Sebald are among the readers captivated by its theory of crystallization—a bizarre and brilliant account of “what happens in the soul” when we begin to fall in love. As Stendhal put it, crystallization “is a mental process which draws from everything that happens new proofs of the perfection of the loved one”; it’s how the desiring imagination progressively transforms the beloved. Another of Stendhal’s later readers, the sexologist Havelock Ellis, put it more bluntly: crystallization is “the mental side of the process of tumescence.”

Stendhal wrote most of Love in the throes of a romantic obsession with Mathilde Viscontini Dembowski, a Milanese acquaintance. The book’s author is obviously going through something: manically insisting, confessing, confabulating, apologizing, calculating. Moving restlessly between diary and philosophy, anecdote and scholarship, he generates a disordered encyclopedia of love—with entries on “thunderbolts” (coups de foudre), the undereducation of women, glances, the way political systems determine the love lives of nations, and the precise conditions that lead to erectile “fiascos.” He keeps trying to reassert analytic rigor—unsuccessfully: “I am trying extremely hard to be dry. My heart thinks it has so much to say, but I try to keep it quiet. I am continually beset by the fear that I may have expressed only a sigh when I thought I was stating a truth.”

Love is full of failed efforts to dry out and quantify intense qualitative experience. In the opening chapters, we are presented with four types of love—and then it turns out that there might be eight or even ten. The process of falling in love is analyzed and enumerated: it has seven stages, and the seventh has three vacillating subphases. A mistress’s beauty can be expressed “mathematically,” measured in so many “units of happiness.” But these units are subjective too; my beloved might promise a hundred units of happiness to me and only three to you. Or, Stendhal suggests, we could think in distances: love is a journey from Bologna to Rome, with four stops along the way.

All of this desperate accounting, or failing to account, adds up to something entirely poetic: the book’s grand metaphor of crystallization. To explain the pathology of the mind in love, its stubborn adherence to idealization and delusion, Stendhal writes,

Leave a lover with his thoughts for twenty-four hours, and this is what will happen:

At the salt mines of Salzburg, they throw a leafless wintry bough into one of the abandoned workings. Two or three months later they haul it out covered with a shining deposit of crystals. The smallest twig, no bigger than a tom-tit’s claw, is studded with a galaxy of scintillating diamonds. The original branch is no longer recognizable.

This exquisite figure for falling in love recurs throughout the book. It’s what the stages of love are tracking: stage 5 is “first crystallization”; stage 7 is “second crystallization.” It’s why my mistress is more lovely to me than she is to you (and why I find even her smallpox scar adorable). It’s the fourth stop on the journey from Bologna to Rome.

Stendhal scatters Love with charming vignettes that illustrate the consequences of crystallization, how it distorts the lover’s vision and impairs his judgment. Each of these little stories is a treasure of psychological insight, both immersing us in the deranged perception of the lover and exposing, mercilessly, what passionate love looks like from the outside.

There’s the one about the arbitrariness of erotic attachment: A young woman hears that her cousin Edward, a distinguished soldier whom she has never met, wants to marry her. He will be arriving any day to propose. When she hears a stranger greeted as “Edward” in church, she falls instantly in love. The real Edward later appears, and she can’t bear the sight of him.

Stendhal shows us that the truth of love is inseparable from love’s illusions.

There’s the legend, from a Provençal manuscript, about abject devotion: A poet has offended his lady. She will only forgive him if he pulls out a fingernail and delivers it to her in a procession of “fifty lovelorn faithful knights.” The poet eagerly assents, and his fingernail is conveyed to the lady with great ceremony, “the lover in the garb of repentance follow[ing] his fingernail at a distance.” He is formally forgiven, and their happiness resumes.

There’s another, from an Arabic manuscript, about love’s absurd sacrifices: A Muslim man who loves a Christian woman converts to Christianity on his deathbed so that he can be with her in the afterlife. In a plot twist worthy of O. Henry, she also dies and also converts. They are eternally divided!

And there’s the anecdote about the self-deceptions demanded by love: A French noblewoman denies an infidelity, despite being caught in flagrante by her lover: “When he pressed, she cried: ‘Oh, I see it all now. You don’t love me any more; you’d rather believe your eyes than what I tell you!’”

Crystallization is most thoroughly elaborated in my favorite part of the book, a short story (or is it an autobiographical reminiscence?) appended to the posthumous edition of 1853. “The Salzburg Bough” recounts the metaphor’s genesis, a trip the narrator takes to the Hallein salt mines with a charismatic Italian friend named Madame Gherardi. At the mines, the friends are enchanted by the salt-encrusted twigs offered to tourists. And another tourist, a young Bavarian officer, is clearly enchanted by Madame Gherardi, becoming ridiculous in her presence.

The narrator has a brilliant epiphany: What has happened to the branch in the salt mine is happening in the imagination of this young man. He is “crystallizing for” her, turning a perfectly human woman into a dream-encrusted ideal. The friends are delighted by the discovery of this metaphor, and it becomes a running joke for the rest of the trip and a punchline in the theater boxes and drawing rooms back home. But there is the subtlest of suggestions that our narrator, for all his ironic detachment, might be a little besotted with his friend too. …

Born in 1783, Stendhal was shaped root and branch by Revolutionary and post-Revolutionary France. Love’s provenance is evident in its philosophical mood; its feminist politics; its belief in the influence of national character, climate, and government on human behavior; even its fascination with the literary history of love. The book is a souvenir of Romantic thought and feeling.

Still, Stendhal’s effort to study the inner experience of love—to hold it up to the light like a crystal and describe its many facets—is something more. Stendhal keeps telling us that his effort is a failure: He is trying unsuccessfully to be mathematical, aiming for truth and expressing “only a sigh.” All the counting and cataloguing of the kinds and phases of love, all the sociological observations, all the love stories we’ve ever heard—none of it makes our own experience of love any less particular and more communicable.

That is because love isn’t an observable object in the world so much as a way of seeing, and beautifully mis-seeing. Stendhal shows us that the truth of love is inseparable from love’s illusions. The figure at the center of his book—a little leafless twig “studded with a galaxy of scintillating diamonds”—makes error beautiful. That figure glories in the lover’s failure to see what others see, to measure accurately and theorize objectively. Of all the imperfect methods for explaining love, metaphor turns out to be the most exact.