Full Idea
The converse Barcan formula does not allow any contingent truths at all to have a truthmaker. Once cannot combine the converse Barcan formula with any truthmaker principle worth having.
Clarification
See in Formal Logic|Modal logic for the Barcan formula
Gist of Idea
The converse Barcan formula will not allow contingent truths to have truthmakers
Source
Timothy Williamson (Truthmakers and Converse Barcan Formula [1999], §3)
Book Reference
-: 'Dialectica' [-], p.265
A Reaction
One might reply, so much the worse for the converse Barcan formula, but Williamson doesn't think that.