Full Idea
A problem with the Barcan Formula is it might be possible for there to exist a ghost, even though there in fact exists nothing that could be a ghost. There could have existed some 'extra' thing which could be a ghost.
Gist of Idea
Barcan Formula problem: there might have been a ghost, despite nothing existing which could be a ghost
Source
Theodore Sider (Logic for Philosophy [2010], 9.5.2)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.239
A Reaction
Thus when we make modal claims, do they only refer to what actually exists, or is specified in our initial domain? Can a claim enlarge the domain? Are domains 'variable'? Simple claims about what might have existed seem to be a problem.