Full Idea
Existence cannot be a matter of degree. If you say there is something that exists to a diminished degree, once you've said 'there is' your game is up.
Gist of Idea
Existence doesn't come in degrees; once asserted, it can't then be qualified
Source
David Lewis (Parts of Classes [1991], 3.5)
Book Reference
Lewis,David: 'Parts of Classes' [Blackwell 1991], p.80
A Reaction
You might have thought that this was so obvious as to be not worth saying, but as far as I can see it is a minority view in contemporary philosophy. It was Quine's view, and it is mine.