In this popu­la­ri­sed article, based on his stu­dy of Pierre Duhem’s archives, the author demons­trates the irre­pla­ceable contri­bu­tion of scien­ti­fic cor­res­pon­dence to research into the his­to­ry and phi­lo­so­phy of science. These archives contain 2,000 let­ters recei­ved by Duhem and 1,000 let­ters sent to his daugh­ter. Their ana­ly­sis sheds light on his rela­tions with Hen­ri Poin­ca­ré, Mau­rice Blon­del and Albert de Lap­pa­rent, and sheds light on the context in which his major works were publi­shed. Some of the let­ters, which might appear to be of lit­tle inter­est, make it pos­sible to recons­ti­tute clas­si­fi­ca­tions or rela­tion­ships. Others refute erro­neous ana­lyses. In the end, 10% of the let­ters are deci­sive. They pro­vide unique assess­ments of his contem­po­ra­ries, cor­rect inter­pre­ta­tions of his work and pro­vide deci­sive evi­dence in his­to­rio­gra­phi­cal debates. Archives are the­re­fore essen­tial to the pro­gress of research. The author would like to thank the archi­vists, past and present, who make this thank­less but fruit­ful work possible.