Some Remarks on Negation and Quantification in Leibniz’s Logic

We argue, with special reference to various passages from Leibniz's Generales Inquisitiones, that the simultaneous pres- ence of two switches concerning the negation operator is the main source of the difficulties raised by many people against his logical investigations. The former switch carries negation from outside to inside of the proposition to which it is applied; the latter is the so- called obversion law (which asserts the equivalence of "non est" and "est non"). Applied in sequence, the two switches transform an out- side (propositional or de dicto) negation into a conceptual (predica- tive or de re) negation. We also investigate the further difficulties which arise from the interplay between these switches and Leibniz's use of indefinite letters.