In the old days it was said that Leibniz endorsed a kind of reducibility thesis about inter-monadic relations. Nowadays we are invited to recognize that no such reduction is available and that Leibniz never intended one. Unfortunately, the issue is obscured by a lack of clarity on the part of Leibniz and his commentators: one is never quite sure whether the reducibility thesis is a claim about relational constructions in a language, a thesis about concepts figuring in propositions expressed by those constructions, a metaphysical doctrine about individual substancescum-accidents, or something else again; and it is never quite clear, given items from any one of those options, in what sort of “reduction” they are to figure as terms. In this chapter we propose a return to the old days. Leibniz was a reductionist about inter-monadic relations, and we shall say in exactly what sense or senses of ‘reduction’ that is so.

Why should a discussion of Leibniz's views of relations figure so prominently in a book on Leibniz on individuation? The reader will discover that Leibniz's reductionism about relations figures centrally in later chapters: his views on relations are integral to the project of modal individuation generally and to the stripe of essentialism to which he was committed in particular; they underlie his views on the Identity of Indiscernibles; they inform his negative views on the role of spatiotemporal relations in constituting the identity and difference of things; and, as we shall argue in a final chapter, they render him prone to a Spinozistic world view.