Why Contingentist Actualists Should Endorse the Barcan Formula

On its usual interpretation, the Barcan Formula—◊∃xB → ∃x◊B—says that, if there could have been something that is such and such a way, then there is something that could have been that way. It is traditionally held that contingentist actualists should—indeed, must—reject the Barcan Formula. I argue that contingentist actualists should—indeed, must—endorse the Barcan Formula, at least assuming a standard, Tarskian conception of truth and truth preservation. I end by proposing a logic for contingentist actualists that validates the Barcan Formula. This logic has the surprising feature of also validating the Converse Barcan Formula, □∀xB → ∀x□B, while still invalidating related formulas—such as □∀x□∃y x = y (NNE)—that contingentist actualists should reject. It does this by employing models with fixed domains but assignments to the identity predicate that vary across worlds.