Full Idea
I claim that mereological composition is unrestricted: any old class of things has a mereological sum. Whenever there are some things, even out of different possible worlds, there is something composed of just those things.
Gist of Idea
Mereological composition is unrestricted: any class of things has a mereological sum
Source
David Lewis (On the Plurality of Worlds [1986], 4.3)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.211
A Reaction
To say the least, a rather unusual usage for the English word 'thing'. I presume that Lewis is in the grip of a slippery slope problem - that there is no way to define the borderline between things and non-things. Presumably 'class' is unrestricted too.