Abstract

Leibniz defends two apparently inconsistent doctrines. On the one hand, he holds that substances are independent entities and that God can, at least in principle, create any possible substance whatsoever no matter what else he creates. On the other hand, Leibniz insists that some possible substances are incompossible with one another and thus cannot coexist. I first discuss three attempts of dealing with this tension in Leibniz’s work that have recently been made in the literature: the logical approach, the lawful approach, and McDonough’s tiling approach. I conclude that none of them solves the problem satisfactorily. I then argue that a modified version of the lawful approach, which also takes into account some of the insights of the two other options, is the most promising strategy. I believe that it is crucial to realize that it is, at least in principle, in God’s power to co-create incompossible substances. I do not take this to imply, however, that such substances can coexist. Since coexistence requires being in the same world, and because not all collections of possible substances are worlds in Leibniz’s sense, I argue that co-creation does not entail coexistence. Thus in order to get clear on Leibniz’s conception of compossibility, I suggest that we must better understand what a possible world is for Leibniz. What does it mean, then, for a collection of possible substances to form a world? I argue, first, that all complete concepts of the relevant possible substances need to have the same laws inscribed in them this is in line with some versions of the lawful approach). In addition, I argue that every complete concept in the relevant collection must also involve the concept of every other substance in the collection. Moreover, I maintain that these two requirements are not reducible to each other and that they are individually necessary and only jointly sufficient for a collection of substances to form a world. Only substances whose complete concepts fulfill both conditions can coexist and thus be compossible.