 Saunders 2010 claims to solve the problem without introducing additional structure into the theory. Working in the Heisenberg picture, he uses appropriate semantics and mereology according to which distinct worlds have no parts in common, not even at early times when the worlds are qualitatively identical. In the terminology of Lewis 1986 (p. 206) we have the divergence of worlds rather than overlap. Wilson 2013, 2020 develops this idea by introducing a framework called “indexicalism”, which involves a set of distinct diverging “parallel” worlds in which each observer is located in only one world and all propositions are construed as self-locating (indexical). In Wilson’s words, “indexicalism” allows us to vindicate treating the weights as a candidate objective probability measure. However, it is not clear how this program can succeed, see Marchildon 2015, Harding 2020, Tappenden 2019a. It is hard to identify diverging worlds in our experience and there is nothing in the mathematical formalism of standard quantum mechanics which can be a counterpart of diverging worlds, see also Kent 2010 (p. 345). In the next section, the measure of existence of worlds is related to subjective ignorance probability.