Black (1952) offers a now-famous conceivability argument against the Identity of Indiscernibles, which appeals to the apparent conceivability of a world containing only two indiscernible iron spheres. In defending haecceitism, a natural strategy is to extend Black’s case by arguing that, not only can we conceive of a world containing only two indiscernible iron spheres, we can conceive of distinct worlds that differ only insofar as these spheres swap their spatial locations. (Jubien (1993: 41–42) discusses an argument of this sort.) A closely related argument holds that we can conceive of a pair of worlds in which some pair of spheres has been replaced by some other pair of spheres. Granted the conceivability of the states of affairs in question, haecceitism follows. (A related line of argument, owing to Adams (1979), is considered below.)