  In the chapter on Leibniz, Erdmann stressed the connection between mathematics and philosophy. He dealt with Leibniz’s logic in the section on the philosophical method and mentioned Leibniz’s definition of “method” as the way to derive all knowledge with the help of “principles of knowledge” (Erkenntnisprinzipien) (Erdmann 1842, 109). These principles are the law of contradiction and the law of sufficient reason. Erdmann quoted Leibniz’s letter to Gabriel Wagner containing the statement that logic is the art to use the intellect; logic is therefore the key to all sciences and arts. According to Erdmann, Leibniz identifies the logical method with the mathematical method regarding it as the true philosophical method. Erdmann furthermore dealt at length with Leibniz’s “mathematical treatment of philosophy” not only because it was important for Christian Wolff and his school, but also “because just this point is usually ignored in presentations of Leibniz’s philosophy” (ibid., 114). He had good reasons for this evaluation because most of the relevant writings became only accessible in his own edition (Leibniz 1839/40). Erdmann discussed Leibniz’s calculi calling them “methodic operations” with data in the “way of calculating”. He mentioned Leibniz’s idea of a character script for the calculus which allows using signs without always having a particular meaning in view. Such “pasigraphy” would eliminate the differences between the languages, but, according to Erdmann’s evaluation, the idea of a universal language was not at the center of Leibniz’s interests. Leibniz’s main point was that “all mistakes in reasoning will at once show up in a wrong combination of characters, and therefore the application of the characteristic script provides a means to discover the mistake in a disputed point like in every other calculation” (ibid., 122–123).