While browsing the Royal Society’s historical archives, Salvatore Riccardo, a historian of science at Italy’s University of Bergamo, stumbled across a spectacular find that he wasn’t even searching for.

The letter, believed to be written by the hand of the scientific giant Galileo Galilei, offers a window into his vexed battle with the Catholic Church, which eventually sentenced him to life-long house arrest for promoting the heresy that the Earth revolved around the Sun.

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“I thought, ‘I can’t believe that I have discovered the letter that virtually all Galileo scholars thought to be hopelessly lost,’” Ricciardo said in the journal Nature, which published an announcement of the discovery. “It seemed even more incredible because the letter was not in an obscure library, but in the Royal Society library.”

Nature explained that the Royal Society had actually been in possession of the letter for 250 years, but it was overlooked by historians over the centuries.

In the letter’s seven pages, Galileo writes to his friend Benedetto Castelli, a mathematician at the University of Pisa, on Dec. 21, 1613, arguing that science should be free from the constraints of theology. He signs the letter “G.G.”

Galileo would eventually be condemned to house arrest by the Inquisition in July 1633 for his beliefs.

In its article, Nature reports that other copies of the letter had existed, but without the original, historians had no way to verify if the letters had been transmitted faithfully. It explains:

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Many copies of the letter were made, and two differing versions exist — one that was sent to the Inquisition in Rome and another with less inflammatory language. But because the original letter was assumed to be lost, it wasn’t clear whether incensed clergymen had doctored the letter to strengthen their case for heresy — something Galileo complained about to friends — or whether Galileo wrote the strong version, then decided to soften his own words. Galileo did the editing, it seems. The newly unearthed letter is dotted with scorings-out and amendments — and handwriting analysis suggests that Galileo wrote it. He shared a copy of this softened version with a friend, claiming it was his original, and urged him to send it to the Vatican.

“It’s so valuable,” Allan Chapman, a science historian at the University of Oxford, told Nature of the letter. “It will allow new insights into this critical period.”

The changes Galileo made to the text shows the excruciating tyranny he lived under, fearful that his strong language could severely cost him:

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