Albert Einstein may have been a genius, but when it came to relationships, he was just as clueless as the rest of us—maybe even more so.

In a letter written near the end of his life to the family of his closest friend, Michele Besso, the great physicist shared poignant regrets about his two marriages and mused on the rarity of combining a “sharp intelligence” with a “harmonious life.” One of a cache of 56 letters that will be auctioned off by Christie’s next month, the letter offers a rare glimpse into Einstein’s feelings about his complicated love life, which included a bitter breakup with his first wife, a second marriage to his first cousin and an affair with a suspected Russian spy.

Einstein and Michele Besso first met as students in Zurich in the late 1890s, and later became close friends while working together at the Swiss federal patent office in Bern. In 1905, when he published the four papers that would forever change our understanding of the universe, Einstein acknowledged only one collaborator: Besso.

But it wasn’t all science talk between the two men. Besso regularly heard reports of Einstein’s personal life, the extent of which is still being revealed through letters held in archives around the world, according to the Times (UK).

Einstein wrote of his troubled marriage to his first wife, Milena Marić, which broke up in part because of his infidelities, including an affair with his divorced first cousin, Elsa, whom he later married. Concerned for the welfare of the couple’s young sons, Albert Jr. and Eduard, Besso subsequently acted as a mediator between Einstein and Milena. At one point, physicist Heinrich A. Medicus wrote in a 1994 scholarly article, Besso and another friend, Heinrich Zangger, helped smooth things over after Einstein canceled a trip back to Switzerland for Christmas after the situation with his wife and children became too tense. Thanks to Besso and Zangger’s intervention, Einstein changed his mind and rescheduled the trip.

In a letter to Besso in July 1916, Einstein thanks his friend for being there for Milena and his kids, while at the same time asserting that separating from her was the only way for him to avert a breakdown. (Being Einstein, he concluded that letter with a joke about quantum theory.)

Again and again, Einstein’s letters to Besso reveal a human side to the great scientist. He worried about money; he discussed his son’s education; he rescheduled plans to meet up. In 1936, after not writing to his friend for several years, he explained the delay by citing the “mathematical imp” sitting on his neck. (Biographers think he also may have been dealing with the painful decline of Elsa, who died of heart and kidney problems later that year.)

After Elsa’s death, and faced with the onset of the Second World War, Einstein threw himself into his work. In a letter from Princeton dated July 10, 1938, he told Besso: “I wouldn’t want to go on living if I didn’t have my work.”

At the time he wrote that letter, Einstein may have been engaged in one of his most intriguing known relationships—his romance with Margarita Konenkova, an alleged Russian spy. Their affair was revealed in 1998, when Sotheby’s auctioned off nine letters written from Einstein to Konenkova between 1945 and 1946.

Although she was married to Sergei Konenkov, a noted Russian sculptor, Konenkova had pursued affairs with the famed composer Sergei Rachmaninoff, among other influential men. She may also have been a spy (code name: Lukas) tasked by Moscow with getting close to J. Robert Oppenheimer of the Manhattan Project.

In fact, Sotheby’s consultant Paul Needham even discovered references to Konenkova’s work for Moscow in “Special Tasks,” a 1995 book co-written by Soviet spymaster Pavel Sudoplatov. As part of her mission to learn about the nuclear program, Sudoplatov wrote, Konenkova was supposed to “influence” Oppenheimer, as well as “other prominent American scientists she met at Princeton.”

Einstein was not directly involved in the Manhattan Project, so it’s unclear what Konenkova might have been hoping to get out of her relationship with him. The letters confirm, however, that she introduced Einstein to the Soviet consul in New York. They reveal no hint that Einstein knew of or suspected Konenkova’s possible spy work, but they do reveal his sappy side. At one point he referred to “Almar,” apparently a pet name for the couple combining their first names, Albert and Margarita.

Though Einstein and Konenkova first met in 1935, it’s unknown whether they began their affair before Elsa’s death in 1936. In any case, his shortcomings as a husband was very much on Einstein’s mind in 1955, when he wrote the final letter in the correspondence, dated March 21, 1955, shortly after Michele Besso’s death.

“This gift of a harmonious life is seldom paired with such a sharp intelligence,” he wrote to Besso’s son and sister just weeks before his own death, at the age of 76. But what I most admired in him as a man was the circumstance that he managed to live for many years not only in peace but in lasting consonance with a wife—an undertaking at which I twice rather shamefully failed.”

In addition to revealing new details about Einstein’s marital regrets, the correspondence with Besso, which spans the years 1903 to 1955, also delves into the theories of special and general relativity, quantum mechanics, unified field theory and a variety of other complex concepts.

“For a non-scientist it was hard—sometimes impossible—to keep up,” wrote Thomas Venning, head of Books and Manuscripts at Christie’s of London, of cataloguing the Einstein-Besso letters. “But the sensation of observing this great mind working at full speed was extraordinary.”