  The most important among these editorial projects was the edition of Leibniz’s philosophical works God. Guil. Leibnitii opera philosophica quae exstant Latina Gallica Germanica omnia (Leibniz 1839/40; cf. Glockner 1932, 59–65) prepared in two volumes by Johann Eduard Erdmann (1805–1892), which included fragments, published there for the first time, containing elaborations of Leibniz’s ideas concerning logical calculi. One of the documents is Leibniz’s 1696 letter to Gabriel Wagner which contains the famous definition of logic or the art of reasoning as the art to use the intellect (Verstand), i.e., not only to evaluate what is imagined, but also to discover (invent) what is hidden. The edition also contains the seminal fragments “Specimen demonstrandi in abstractis” and “Non inelegans specimen demonstrandi in abstractis” (Leibniz 1839/40, 94–97), the last with the algebraic plus-minus calculus, i.e. a central specimen of Leibniz’s various attempts to create logical calculi working with the two “constituting” operations “gathering together”, symbolized by +, and “taking away”, symbolized by – (cf. Leibniz 1999, no. 178).