Journal of the History of Philosophy

This collection brings together nine essays addressing the problem of compossibility in Leibniz's system of thought. The problem of compossibility is that of determining by what mechanism two independently possible substances are jointly possible (or not), and thus do (or do not) form part of the same possible world. The collection opens with an excellent introduction to the terrain, reviews established approaches to the problem, and will be extremely helpful to both those new to, and those familiar with, Leibniz.

Adam Harmer's essay provides a detailed analysis of the "world apart" doctrine, for which he considers three different interpretations: causal, ontological, and phenomenal, as well as weak and strong forms of each. Harmer concludes that Leibniz's world apart doctrine entails the weak versions of all three interpretations; yet this does not impact which of the three main approaches to compossibility (the logical, lawful, and cosmological) are correct. Mogens Lærke, in the process of arguing for the logical approach, interprets world apart (correctly in my opinion) as entailing only the causal independence of monads at the metaphysical level. It has no affect on their conceptual dependence on each other, nor on the causal dependence at the phenomenal level that is grounded upon their conceptual dependence.

Sebastian Bender argues that in order to understand compossibility we need to first understand the concept of a possible world. Focusing on world apart, he goes on to make a distinction between coexistence and co-creation. The possible substances in the former exist together in a world, whereas the latter are mere collections of substances that do not form a world. Bender further claims that substances are compossible if and only if their concepts contain the same laws of the universe and represent all of the other substances in the universe. Note that this does not mean God must create all members of the world, for Bender, because of the world apart doctrine. Yet this move seems to be in tension with another seeming Leibnizian requirement on worlds—the denial of transworld identity—that a possible substance cannot exist in more than one possible world.

Here we see a common problem I would like to have seen addressed in some of these essays: the doctrine of world apart and that of the "denial of transworld individuals" pull in opposite directions. World apart requires the existence of solipsistic worlds (worlds with only one substance), wonky worlds (worlds where each possible substance is dancing to its own tune), or something like a world yet not a world, which seems to violate the ban on transworld individuals. At one point it was popular to stress the ban. Now it seems the pendulum has swung in favor of world apart. [End Page 735]

The contributions to this volume by Thomas Feeney and Julia Joráti form a nice pairing. Both argue that, to get clear about compossibility, one must focus on the different attributes of God and the way these attributes are related to possible worlds. Joráti argues that God's intellect and power are governed by the principle of contradiction so that, relative to these attributes, any collection of possibles are possible (compossible?). Yet relative to God's wisdom, which is governed by the principle of sufficient reason (PSR), only harmonious collections of possibles are compossible. By 'harmonious,' Joráti means that the world is governed by the PSR in such a way that the changes in one substance's perceptions are explained by changes in another; and thus, the phenomena of these worlds are causally connected.

The essay by Gregory Brown meticulously argues that in every possible world there is a plenum, yet void space is not impossible (full disclosure: I had the pleasure of working with Greg on issues Leibnizian during my master's work, and he served as an advisor on my PhD committee). Brown claims that Leibnizian considerations of the principle of the...