According to Lewis, any pair (or, more generally, plurality) of concrete (he doesn't even restrict it this way) of objects has a mereological sum. Now, suppose that x and y are concrete objects in worlds w1 and w2 respectively. Let z be the mereological sum of x and y. According to Lewis, worlds are maximal spatiotemporally connected sums of objects. Now, here are some plausible principles:
- Spatiotemporal connection is transitive and symmetric.
- If a is spatiotemporally connected to a part of b, then a is spatiotemporally connected to b.