After an evening spent flitting between cabarets and bars, a visitor to Weimar Berlin could walk a block or two the next day and find themself at the threshold of the Institute for Sexual Science. Inside, despite the classical furniture and marble floors, this mansion looked to the future. An exhibition space featured sexual curios from around the world, including phalluses from as far as Japan and Central Africa, and a glass-topped display of fetishes—among them, a collection of women’s underwear that one Prussian soldier had worn under his uniform.

In another section of the institute, a “Wall of Sexual Intermediaries” showcased photographs of individuals who embodied sexual difference. Here, our visitor might begin to feel a rising sense of familiarity: The image of “Anna H., a waitress,” might bear a certain resemblance to someone who had served them drinks only hours ago. Or, over there—that drawing of a tableau vivant looks awfully similar to the bawdy onstage performance from another stop the night prior.

These were the blurred lines between after-hours Berlin and the Institute for Sexual Science. Founded by the sexologist Magnus Hirschfeld shortly after the fall of the Kaiserreich, the institute was a museum that sought to demystify sex—not just through curation and research, but also through its open doors for those seeking refuge. Daniel Brook’s new biography, The Einstein of Sex: Dr. Magnus Hirschfeld, Visionary of Weimar Berlin, traces the pioneering work of Hirschfeld at the institute and beyond.

Hirschfeld was an intellectual star and activist who argued that sex, gender, and race were not fixed categories but were instead defined by incredible variation. His thought and practice, Brook writes, rendered him “exhibit A for the claim the republic had gone too far” once the National Socialist German Workers Party seized power. Exiled from Germany in 1933, Hirschfeld spent his final years in France among his fellow émigrés until his death in 1935. His life has since been forgotten.

Hirschfeld was born in the Prussian spa town of Kolberg in 1868 to a well-to-do Jewish family. Decades before, the Napoleonic Wars transported some of the ideals of the French Revolution to Prussia. Hirschfeld’s parents were the first generation to benefit from this liberal turn, when one could be publicly Jewish, participate in civil society, and even nearly reach the highest echelons of professional life. In this open-minded environment, Hirschfeld became curious about the wider world from a young age: At 10 years old, he set out for an unsuccessful trip by foot from Kolberg to Paris. As a student, he was drawn to the Greeks and their tolerant treatment of same-sex relations. Early on, Hirschfeld knew he was attracted to men. He must have found a welcome reprieve in ancient texts—for example, Aristophanes’ account of the origins of gay men and lesbians in Plato’s Symposium—because in Bismarck’s Germany, by contrast, normalsexuell reigned, and sex between men was a crime punishable by up to five years in prison.

In between his seaside promenades, Hirschfeld mused on a future in either journalism or medicine, eventually opting to pursue the latter at Berlin University. He received his license in 1892, but before officially launching his career as Dr. Hirschfeld, he decided to make good on his adolescent ambition to see the world and so embarked on an intercontinental trip that would shape his capacious understanding of sexuality. In Chicago—whose streets were a “meat market” despite “sodomy” being a felony in America, Brook writes—Hirschfeld ventured to Clark Street, “famed for its drag performers and sex workers.” In Tangier, he found the seaport to be a cruising site and visited the hammans, or sex-segregated public bathhouses, remarking that “homosexuality blossoms here abundantly”—an observation that could have summarized his trip in its entirety.

After his journey through the international sexual underworld, Hirschfeld returned to open a medical practice in Magdeburg, a city in central Germany. His embrace of naturopathy, which privileged natural remedies over invasive procedures, set his practice apart from the German medical establishment. And once his sexual orientation became an open secret, it didn’t take long for Hirschfeld to become known “as the doctor you could trust with your sexual secrets.” He soon developed his thesis that human sexuality existed on a continuum. In 1896, he published the gay rights pamphlet “Sappho and Socrates,” which challenged the argument that homosexuality stemmed from childhood trauma—as Hirschfeld’s contemporary Sigmund Freud proposed—and theorized that sexual orientation is “inborn” instead of a choice. He then convened the world’s first gay rights organization, the Scientific Humanitarian Committee, which organized a petition against the law that made gay sex illegal in Germany—Paragraph 175 of the German Penal Legal Code. Though it failed on the floor of the German parliament, Hirschfeld succeeded in turning the penal code into a “question of the day” as well as his lifelong crusade.

Hirschfeld continued to minister to patients as he published a number of studies and became Germany’s most public advocate for more liberal attitudes around sex. He supported the creation of permits that protected people whose gender did not appear to match their clothing against claims of “disorderly conduct” or “public nuisance” when stopped by the Berlin police. He scoured gay institutions, including the ball scene, as research for Berlin’s Third Sex, relying on his observations to “depict everything as it is.” He also wrote The Homosexuality of Men and Women, his 1,000-plus-page “magnum opus,” as Brook describes it, tapping into his travels and the “global network of gay informants he’d cultivated over the years” to expound on his “born this way” theory.

Then the 1918 November Revolution put an end to the imperial regime of the Kaiserreich and led to the creation of Germany’s first democracy, the Weimar Republic. Held together by a coalition led by the Social Democratic Party, the republic aspired to a new order based on universal rights and liberalism. In short order, a wartime ban on dancing was repealed, leading to raucous parties that stretched into the New Year and then into the summer. The new government lifted press censorship, protected freedom of speech, and established an eight-hour workday. “In the storm of revolution, with one blow full citizen rights have fallen into our lap,” wrote the feminist Marie Stritt about the sudden obtainment of women’s suffrage. The world appeared transformed and the horizon endless. “Our hour of freedom has come,” the editor of the popular gay magazine Friendship declared.

Hirschfeld created the Institute for Sexual Science amid all these heady changes in German society. On its opening night in 1919, he described the institute as a “child of the revolution.” The reorganization of property after the fall of Imperial Germany enabled Hirschfeld to purchase the mansion that would house the institute. The kaiser had intended to use the building to expand the opera house, but as Hirschfeld noted, “war and revolution ended those plans.” Hirschfeld lived on the ground floor with his life partner, Karl Giese, while the library housed some 20,000 volumes of sexual science and literature. Sex education—underpinned by a sort of inverse eugenics that saw interracial marriage as necessary for progress—was dispensed in an auditorium during the evenings. The world’s first gender-affirming surgery was completed at the institute, and psychotherapy was offered on a sliding scale to patients experiencing gender dysmorphia. The Hirschfeld Museum (a “labyrinth of human passions and aberrations!,” as one in-house physician described it) was the most popular part of the institute, including among international tourists benefiting from the weakened German mark.

Hirschfeld eventually applied the same theory of relativity to gender as he had to sexuality. He calculated the existence of 43 million gender possibilities, writing:

In each person there is a different mixture of manly and womanly substances, and as we cannot find two leaves alike on a tree, then it is highly unlikely that we will find two humans whose manly and womanly characteristics exactly match in kind and number.

In other words, Hirschfeld argued that gender exists on a continuum so expansive that it almost resists categorization, and he granted this conclusion a scientific sheen through his mathematical analysis.

In The Einstein of Sex, Brook writes that Hirschfeld also advocated for the legalization of abortion and no-fault divorce and supported women’s suffrage. Brook’s portrait of Hirschfield, however, does not probe his connection to the broader transformation of gender during the Weimar era. As Helen Boak argues in Women in the Weimar Republic, the war effort upended the sexual division of labor—the first crack in the traditional gender order. The republic then brought easier access to birth control and abortion and the possibility of sexual liberation. (According to Boak, the majority of the 3,000 visitors to the institute during its first year sought birth control information.) Women began to enter public spaces more, and a distinct Weimar figure emerged from this opening of society: the “new woman.”

The new woman could be seen in political propaganda and advertisements. She could also be found in the interwar German art movement known as the New Objectivity. When the artist Otto Dix encountered Sylvia von Harden, a journalist and poet, on the street in her androgynous dress, he claimed she “represented a whole epoch.” In Dix’s painting Portrait of Journalist Sylvia von Harden (1926), the eponymous subject sits alone in Romanisches Café, a popular meeting place for artists and intellectuals in Berlin. She wears a pageboy haircut—a signal of crisis or liberation, depending on your perspective on the November Revolution. Many of her features appear enlarged as a shorthand for masculinity: One of her hands rests on her lap, the other clasps a cigarette. Her jawline and eye sockets look stretched and overblown, given a distorted prominence. One of her stockings has become loose near the hem of her dress. Von Harden had refashioned her gender through the new economic and sexual freedoms that came with the Weimar Republic. Would Hirschfeld have considered her one of his 43 million gender possibilities, one leaf unlike all the rest?

In the later chapter of his life, Hirschfeld sketched out his thinking around race as he embarked on a second world tour. In New York City, he met with the Harlem Renaissance and civil rights giants Langston Hughes and James Weldon Johnson. Hirschfeld grew skeptical of what Americans would have referred to as the “one-drop rule” as a determiner of race. When he arrived in Shanghai, he struggled to see how there could be a single “Semitic” race when the Chinese Jewish community he encountered appeared just like their neighbors. In Manila, a further stop along his journey, he and his new partner-protégé, Li Shiu Tong, were stopped by a border agent who could not believe that the Hong Kong–born Li could be British. The fallacy of race as a biological reality seemed to present itself every way he turned.

Brook argues that Hirschfeld saw race as an invention, as he outlined in his posthumously published Racism (1938), in which he likened race to “such an invention as poison gas.” However, scholars like Laurie Marhoefer have argued that Hirschfield’s theory of sexuality rests on the existence of race. For instance, Hirschfield wrote that “the uniform aspect of homosexuality in all races and under all skies has been for me convincing proof of its biological causation”—a line that reads as if it could have been penned right after returning from his first world tour. In parts of Racism, it seems that Hirschfeld couldn’t fully escape the concept of race: He momentarily repudiates it, only for it to boomerang back. His study concludes on the not-so-high note of calling for an international committee to examine whether “the racial problem…has any scientific basis, or is wholly fallacious.”

As Hirschfeld was writing Racism in the early 1930s, the Nazi Party was continuing its political ascent in the polls and on the streets, emboldened by an economy in shambles, mass unemployment, and an ambient conspiracy theory that characterized the 1918 revolutionaries as national traitors. Adolf Hitler was appointed chancellor in a backdoor deal in January 1933, and his worldview and that of the party—hateful, bigoted, supremacist—soon became the law of the land. The most prominent of their early policies was one establishing that a person in Germany was considered Jewish if one of their grandparents had listed their religion as such on a birth or marriage certificate in the 19th century. (Peter Fritzche writes in Hitler’s First Hundred Days that “everybody learned the lesson of race when they tracked down their grandparents.”)

As Germans rushed to certify their supposed ancestry—with employment in the civil service and then life or death hinging on a fraction—Hirschfeld’s ideas posed a threat to the pseudo race science of the National Socialists. The Nazis revoked his German citizenship in 1933 after deeming him a “Judeo-Bolshevik.” They accused him of luring Germany’s youth into “homosexual seduction,” thereby leading the nation into degeneracy and decay. Meanwhile, German publishers also deemed Hirschfield’s work dangerous and refused Racism when it was submitted. The book would be released three years after Hirschfeld’s death in 1935, when two of his friends translated it and found a publisher in Victor Gollancz, a British son of German Jewish immigrants.

The Nazis similarly deemed the Institute for Sexual Science to be a “breeding ground of dirt and filth” hidden behind a “scientific cloak.” So they ransacked it. Members of the Nazis’ Institute for Physical Fitness—all young men who were part of the Hitler Youth—poured ink over the institute’s manuscripts and tore photographs from its walls. Later the same day, Sturmabteilung storm troopers arrived and seized troves of books—works by Freud, Oscar Wilde, and Margaret Sanger among them. Four days later, on May 10, 1933, the Nazis burned the looted volumes at Opernplatz before an audience of 40,000. While there were smaller book burnings in university towns later the same day, this one was the first and the largest.

Brook notes that the book burning is one of the most remembered watersheds in the Nazis’ violent rise to power, but lesser known is the fact that it was the institute’s books in flames. Faced with threats to his life, Hirschfeld left Germany and went on the move in Europe, communing with his fellow German Jewish exiles as he continued to write and deliver the occasional lecture. One day shortly before his death in the French Riviera, he visited a cinema in Paris and, as was typical for the time, watched the newsreel shown before the feature film started. As Brook writes, Hirschfeld saw “the sharp contrast between the dark, drizzly Berlin night and the hot white of flames” consuming the institute’s volumes, and he wept.

Today, at Bebelplatz (formerly Opernplatz), an art installation titled The Empty Library sits at the site of the book burning. Visitors can peer through a window planted amid the cobblestones and see the bare white shelves of a library below, a testament to all the knowledge lost. This empty underground library works as a nested metaphor—a symbol, too, for what we continue to forget. Among the empty shelves, there is no mention of Hirschfeld or the institute.