 The received view is that the objects of belief are proposition and propositions are sets of possible worlds. But what are these supposed to be? This is a rather difficult question (see the entry on  possible worlds).  On one picturesque view, a possible world is a complete description of an alternative reality. To pick out a possible world is to specify–in a way careful to avoid contradiction–every fact that holds in some possible reality that is not necessarily our own. On this view, the set of  all  possible worlds \(W\) is like a giant library that contains the complete history of every possible reality. The actual world picks out the volume that corresponds to our own reality.