Essential Differences. Or, an Exercise in Symptomatic History of Philosophy

In a letter to Conring dating March 1678, Leibniz produces the following rhetorical question: “Formas substantiales quis neget, id est differentias essentiales corporum?” (“Who would ever reject substantial forms, that is to say, the essential differences of the bodies?”). The chapter tries to clarify this unusual locution, as the possible symptom of some more general and, perhaps, more interesting issue. The two distinct puzzles that are posed by this quote–one concerning the first two words, namely “essential differences”, and the other concerning the specification “corporum”, ‘of the bodies’–will be considered in the light of Leibniz’s reflections of the years 1677–78 on incorporeal substances, and on essences and reality, that culminate with the theory of ‘individual substances’ presented by Leibniz in the Generales inquisitiones and the Discourse on Metaphysics at the denouement of this elaboration period. Due attention is given to the definitions of “essential differences” available in some relevant Scholastic sources, both from 13th century high Scholasticism and from late Scholastics, as the relevant linguistic and conceptual terms of reference for a better understanding of Leibniz’s 1678 expression.