 It is not necessary–and perhaps unhelpful–to think of possible worlds as total metaphysical possibilities. At this extremely fine level of granularity, each possibility specifies an infinity of obscure and uninteresting details. But context usually determines which features of the world we can take for granted; which we are uncertain about but would prefer not to be; and which are of no interest. For example, Sophia may be interested in the identity of the next mayor of Vienna, but whether they are left or right-handed is of no importance. For our purposes, a possible world is a complete specification of all and only those features of the world that are relevant given the context. The set \(W\), therefore, is the set of all contextually relevant epistemic possibilities. Narrowing down the set of possibilities to an individual \(w\in W\) would completely settle some interesting question under discussion. A proposition \(P\subseteq W\) is a  set of possible worlds, i.e. it is a partial specification of the way the world is. To be certain that \(P\) is true is to be certain that the actual world is among the set of worlds \(\{ w : w \in P \}\) since \(P\) is true in a possible world \(w\) iff \(w\in P\).