Parts and Partitions

We use some ideas of David Lewis firom modal metaphysics ‐ in particular the concept of duplication and the Principle of Recombination—together with Lewis's theory of subject matters as partitions of the set of worlds, to illuminate the perennially elusive topic of logical subtraction, systematic theorising about which is in dire need of a non‐standard notion of logical independence. A paradigm case of the desired notion emerges as the kind of independence which obtains between statements which are in Lewis's sense ‘entirely about’ mereologically disjoint spatial regions.