 The entry on the  possibilism-actualism debate  includes a supplement with a proof of BF against the background of (B). But this formula is deemed unacceptable by many philosophers. Consider my plans to build a bookcase. Even if I never manage to build one, it is certainly possible for me to build one. So, it is possible for there to be some bookcase I have built. By the Barcan Formula, it seems to follow that something is possibly a bookcase I build. The Barcan Formula does not settle whether or not this object is in fact a concrete object, much less a bookcase one may in fact use to store one’s books. In fact, to deny this is one way to come to terms with the Barcan Formula.