 Essential to Weinberg’s argument is an appeal to the principle of indifference, applied to a class of  observers.[44]  We should calculate what we expect to observe, that is, as if we are a “random choice” among all possible  observers.[45]  Bostrom (2002) argues that indifference-style reasoning is necessary to respond to the problem of “freak observers”. As Bostrom formulates it, the problem is that in an infinite universe, any observation \(O\) is true for some observer (even if only for an observer who has fluctuated into existence from the vacuum). His response is that we should evaluate theories based not on the claim that some observer sees \(O\), but on an indexical claim: that is, we make the observation \(O\). He assumes that we are a “random” choice among the class of possible observers. (How to justify such a strong claim is a major challenge for this line of thought.) If we grant the assumption, then we can assign low probability to the observations of the “freak” observers, and recover the evidential value of \(O\).