Full Idea
The converse Barcan is at least plausible, since its denial says there is something that could fail to meet a condition when everything met that condition; but how could everything meet that condition if that thing did not?
Gist of Idea
Converse Barcan: could something fail to meet a condition, if everything meets that condition?
Source
Timothy Williamson (Truthmakers and Converse Barcan Formula [1999], §3)
Book Reference
-: 'Dialectica' [-], p.264
A Reaction
Presumably the response involves a discussion of domains, since everything in a given domain might meet a condition, but something in a different domain might fail it.
Related Ideas
Idea 15132 The Barcan formulas fail in models with varying domains [Kripke, by Williamson]
Idea 15135 If the domain of propositional quantification is constant, the Barcan formulas hold [Williamson]