Substance & Individuation in Leibniz (review)

This close engagement with Leibniz's modal metaphysics is as rewarding as it is challenging. Crisply written and tightly argued, the book aims to achieve a balance between what the authors describe as their historical objective—to ascertain Leibniz's views on individuation against the backdrop of scholastic philosophy—and their philosophical goal—to investigate the issue of individuation more generally by construing Leibniz as a mediary between scholastic and modern metaphysics (4). The result is an original, if at times contentious, reinterpretation of some of the central tenets of Leibniz's philosophy.

Since a full discussion of the work is beyond the scope of this review, we focus on one principal line of argument that supports much of what the authors say about Leibniz. Although in early texts Leibniz occasionally shows sympathy for a relational view of individuation (59-63), his dominant position (developed in the seminal Disputatio metaphysica) is that a substance's principle of individuation is wholly internal to it. For the authors, this becomes a benchmark for the interpretation of Leibniz's later philosophy. On their reading, Leibniz's account of individuation rests on two main theses: "the requirement of separability" (that the existence of any substance is metaphysically independent of that of any other substance except God) and "the requirement that whatever individuates a substance must be wholly internal to it" (87).

These theses are heavy with consequences. Concerning the ontological status of relations and relational properties (or extrinsic denominations) the authors convincingly argue that Leibniz embraces a reductionist position, according to which relational properties supervene on a substance's intrinsic accidents, i.e., its perceptions and appetitions. Monadic facts about substances suffice to determine all relational facts, and the latter cannot change without a change in the former (though the converse does not hold). Emphasizing Leibniz's statement that God sees in a complete concept the "foundations" of all that can be predicated of a substance, the authors add to this the more controversial claim that extrinsic denominations are not included in the complete concept of an individual substance (83-4). Thus, the identity of any substance is defined by the totality of its intrinsic denominations, and these in turn determine the relations that hold among substances.

The centerpiece of the book addresses the modal dimension of Leibniz's theory of individuation. Here the authors argue that Leibniz adheres to what they call "strong essentialism" rather than "superessentialism." The former, like the latter, holds that a substance could not have a complete concept different than the one it has. However, as a "strong essentialist" Leibniz denies that all of a substance's properties are essential to it. Based on their reading of Leibniz's doctrine of relations, the authors maintain that all and only intrinsic denominations are essential to a substance. From this they infer that Leibniz allows for the transworld identity of substances: the same substance can exist in different possible worlds, worlds distinguished by the fact that they contain different constituent substances. This is possible, according to the authors, because the "laws of harmony" impose no constraint on which substances can coexist in a world. A possible world is defined by the collection of individuals supposed to exist in it, and laws of harmony are merely relational truths supervening on those substances' intrinsic accidents (109).

The authors are candid in admitting that their interpretation cannot be reconciled with all of Leibniz's texts. Their goal instead is to give Leibniz the best he can get, metaphysically speaking: "Our efforts here are to show that the architectonic of Leibniz's own metaphysics is properly captured by strong essentialism, that the important and deepest threads of his thinking better hang together on that construal" (103). Yet one might take issue with their view of what count as Leibniz's "deepest" thoughts. Surely it is a...