Full Idea
The Converse Barcan says nothing passes out of existence in alternative situations. The Barcan says that nothing comes into existence. The two together say the same things exist no matter what the situation.
Gist of Idea
The Barcan says nothing comes into existence; the Converse says nothing ceases; the pair imply stability
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 4.9)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.113
A Reaction
I take the big problem to be that these reflect what it is you want to say, and that does not keep stable across a conversation, so ordinary rational discussion sometimes asserts these formulas, and 30 seconds later denies them.
Related Idea
Idea 13729 The Barcan corresponds to anti-monotonicity, and the Converse to monotonicity [Fitting/Mendelsohn]