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Full Idea
That there is just one necessary existent is surely false, for if x is a necessary, {x} is a distinct necessary existent.
Gist of Idea
A thing can't be the only necessary existent, because its singleton set would be as well
Source
Timothy Williamson (Truthmakers and Converse Barcan Formula [1999], §1)
Book Ref
-: 'Dialectica' [-], p.257
A Reaction
You would have to believe that sets actually 'exist' to accept this, but it is a very neat point.