 P3 says that the following is true of every proposition \(p\) which can be true: for every world where \(p\) is true, there are two sets of facts \(G\) and \(H\) such that (i) in \(w\), all members of \(G\) exist and none of \(H\) do, and (ii) in every world where this is the case, \(p\) is true. That principle is rather ugly, but it does what it should do.