The document itself is surprising, but so is its discovery. Last August, Salvatore Ricciardo, a postdoctoral science historian at the Italian University of Bergamo, had with much luck exhumed the letter signed by Galileo from the archives of the Royal Society. Historians had been dreaming of getting their hands on it for 400 years.‘I could not believe that this letter was the one that practically every Galilean scholar thought was lost forever,’ Ricciardo said. Several weeks after his discovery, the researcher is still stunned. ‘It was not in an obscure library but before our very eyes, on the shelves of the Royal Society!’Two versions of the same letterAn illustrious mathematician, physicist and astronomer, Galileo lived during the sixteenth century. Like many scientists of his time, he suffered frequent attacks from religious institutions. In 1633, he was arrested by the Inquisition, a jurisdiction of the Catholic Church charged with condemning heresies. The crime of the Italian scientist? Having notably defended heliocentrism which was first theorised by Nicolas Copernicus 70 years earlier.This theory placed the sun at the centre of the universe and thus went against the religious doctrine that it was the Earth that was at the centre of the universe. Galileo was threatened for about twenty years but it is a letter written in 1613 which really ignited the conflict. Sent to his friend Benedetto Castelli, a mathematician at the University of Pisa, Galileo expressed his doubts about the Catholic church.For many centuries, historians had established that two versions of this same letter existed. The first, that received by the Inquisition, stated in a hostile tone that scientific research should be liberated from theological doctrine. Galileo wrote that, according to him, the rare astrological events described in the Bible should not be taken literally.The scholar argues that the scribes would have simplified these descriptions so that they could be understood by ordinary people. He also wrote that the heliocentric model of the Earth orbiting the Sun is not incompatible with what is written in the Bible.Erasures and annotationsA second letter, which was thought to be lost, was said to have been written in a more conciliatory tone. Nobody had any trace of it, however, until today. This letter rediscovered by Salvatore Ricciardo, marked with several erasures and annotations, shows that Galileo did try to add some nuance to his words, just as he had during his trial before the Inquisition.The copy received by Rome in 1816 was therefore not the original. The missive was most likely falsified by a cleric who wanted to harm the scientist. In this annotated version, Galileo steps his comments back by mitigating the more divisive remarks. For example, in his first version, he refers to a passage from the Bible as ‘false, if we rely on the literal meaning.’ The word ‘false’ has been scratched out and replaced by ‘appears different from the truth.’These nuances change the game for historians. This lost letter was in the possession of the Royal Society of London for 250 years. It seems that it was a bad indexation of the document that caused it to be lost into oblivion. The experts are now trying to trace the letter’s path before it came into possession of the institution in London.