Full Idea
Many surfaces are equally good candidates to be boundaries of a cloud; therefore many aggregates of droplets are equally good candidates to be the cloud. How is it that we have just one cloud? And yet we do. This is Unger's (1980) 'problem of the many'.
Gist of Idea
We have one cloud, but many possible boundaries and aggregates for it
Source
David Lewis (Many, but almost one [1993], 'The problem')
Book Reference
Lewis,David: 'Papers in Metaphysics and Epistemology' [CUP 1999], p.164
A Reaction
This is the problem of vague objects, as opposed to the problem of vague predicates, or the problem of vague truths, or the problem of vague prepositions (like 'towards').