Full Idea
Stalnaker holds that there could have been people who do not actually exist, but he denies that there are things that could have been those people. That is, he denies the unrestricted validity of the Barcan Formula.
Gist of Idea
To say there could have been people who don't exist, but deny those possible things, rejects Barcan
Source
report of Robert C. Stalnaker (Counterparts and Identity [1987]) by Ian Rumfitt - The Boundary Stones of Thought 6.2
Book Reference
Rumfitt,Ian: 'The Boundary Stones of Thought' [OUP 2015], p.159
A Reaction
And quite right too, I should have thought. As they say, Jack Kennedy and Marilyn Monroe might have had a child, but the idea that we should accept some entity which might have been that child but wasn't sounds like nonsense. Except as fiction…..