Full Idea
Barcan's axiom says if there can be something that has a certain property, then there is something that can have that property. It and its converse are not obviously correct or incorrect. They claim that it is non-contingent what individuals there are.
Gist of Idea
If a property is possible, there is something which can have it
Source
Timothy Williamson (Laudatio: Prof Ruth Barcan Marcus [2011], p.1)
A Reaction
Williamson defends the two Barcan formulas, but the more I understand them the less plausible they sound to me.