The contribution is centred on the analysis of some letters Diderot addressed to Sophie Volland in autumn 1760 about the discussions which took place in the “salon” of baron d’Holbach at Grandval. Focusing in particular on a letter dated 26 October 1760 where one volume of the Histoire universelle is mentioned, this article proves that the reference was not to the Essai sur le mœurs of Voltaire – as usually believed until now – but to the Universal History, an English historical compilation, whose volumes of the Modern Part begin to be published in 1759. Undoubtedly, it is from volume V of this part of the work that the episode of the Persian history on which Diderot concentrates the attention in his letter dated 28 October is drawn. The identification is also confirmed by the fact that an Irish “contrefaçon” -entirely identical to the English original edition - of this work was in the library of baron d’Holbach. In the light of this finding, more attention should be deserved to the 1968 study by J. Lough on the Universal History, considered as the source of some historical articles on different religions written by d’Holbach for the Encyclopédie, the aim being to support and confirm Lough’s results. This article throws some light on how d’Holbach and his friend Diderot worked on the information contained in a historical compilation such as the Universal History, which was going to become a few years later also the main source especially for the first books of the Histoire des deux Indes by the “abbé” Raynal.