 For \(R_{a}\) to faithfully represent a relation of indistinguishability, what worlds should it relate? If Zoe was plunged in \(w_{1}\) for example, could she tell that she is not in \(w_{2}\)? No: the relation of indistinguishability is symmetricif one cannot tell a from b, neither can one tell b from a. That a relation is symmetric is typically drawn by omitting arrow-heads altogether or by putting them in both directions: