Two French philosophers known more for their feud than their friendship may have been more amicable than previously thought.

A newly discovered letter between Jean-Paul Sartre and Albert Camus reveals how the pair had an early friendship that dated back to before they became divided over their opposing views toward violence in the 1950s, Agence France-Press reports.  

"My dear Sartre [...] let me know when you return, we will spend a free evening together," Camus begins in his handwritten letter. The undated letter was discovered in an original edition of Sartre’s writings, of which only 60 copies were published by two bookstores in Orléans, France, the Guardian reports.

Two librarians, Hervé and Eva Valentin, found the letter in September while preparing for an exhibition on Camus' life to mark his 100th birthday.

"We have done some bad work, my friends and I; so bad that I'm sleeping badly," Camus wrote, referring to a failed attempt to stage one of his plays.

Camus probably wrote the letter sometime between 1943 and 1948. "The letter is very important because it shows that despite what some writers have said, Sartre and Camus had a close friendship," Ronald Aronson, a Sartre specialist, told the French newspaper Libération.

The two French intellectuals had little in common. Sartre came from a bourgeois background while Camus was raised in Algeria by working-class parents. Sartre was drawn to politics after his experience as a prisoner of war, and Camus wanted to advocate for Algeria’s poor. After the war, the pair became France’s leading existentialists -- and friends -- until they parted ways in 1952, when Sartre converted to communism and Camus decided to be "neither victim nor executioner" and denounced the Soviets.

But the details surrounding the short letter remain a mystery. "No response, no reliable testimony sheds light on this mystery, all correspondence between the two men has been destroyed," Eva Valentin told the AFP.

The letter will be displayed from Sept. 3 to Sept. 8 at a Camus exhibition in Lourmarin, roughly 40 miles north of Marseilles.