Think of a cloud — just one cloud, and around it a clear blue sky. Seen from the ground,
the cloud may seem to have a sharp boundary. Not so. The cloud is a swarm of water droplets.
At the outskirts of the cloud, the density of the droplets falls off. Eventually they are
so few and far between that we may hesitate to say that the outlying droplets are still
part of the cloud at all; perhaps we might better say only that they are near the cloud.
But the transition is gradual. Many surfaces are equally good candidates to be
the boundary of the cloud. Therefore many aggregates of droplets, some more inclusive and
some less inclusive (and some inclusive in different ways than others), are equally good
candidates to be the cloud. Since they have equal claim, how can we say that the cloud is
one of these aggregates rather than another? But if all of them count as clouds, then we
have many clouds rather than one. And if none of them count, each one being ruled out
because of the competition from the others, then we have no cloud. How is it, then, that
we have just one cloud? And yet we do. (Lewis 1993: 164)
as cited at
http://plato.stanford.edu/entries/problem-of-many
clouds coming soon...