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Simon Saunders and David Wallace are attempting to use a modified form of David Lewis's analysis of personal fission to support the thought that prior to undergoing Everett branching an informed subject can be uncertain about which outcome s/he will observe. I argue that a central assumption of this seductive idea is questionable despite appearing innocuous and that at the very least further argument is needed in support of it. Simon Saunders and David Wallace suggest that a subject about to knowingly undergo branching in the Everett multiverse can be understood to be uncertain about what s/he will experience if David Lewis's view of the transtemporal identity of persons through fission is modified (Saunders and Wallace 2007, Wallace 2005a: sec. 3.4, 2005b:14; Lewis 1976). Wallace has what can appear to be an independent argument for pre-measurement uncertainty in making intelligible Hugh Everett III's ‘relative state ’ interpretation of quantum mechanics; it is an argument from linguistic interpretative charity. I shall not be challenging that argument here but I shall have something to say about it in relation to the Lewis-based idea. Wallace’s 2005 papers only make brief mention of this idea but it is the focus of attention of Saunders ’ and Wallace’s (2007). Lewis used the world-tube (or ‘worm’) version of transtemporal identity which takes a persisting object to be an aggregate of temporal parts or ‘stages’. For personal identity, the cement which holds the aggregate o