  In "Defeating Dr. Evil with Self-Locating Belief", Adam Elga proposes and
defends a principle of indifference for self-locating beliefs: if an individual
is confident that his world contains more than one individual who is in a state
subjectively indistinguishable from his own, then he should assign equal
credences to the hypotheses that he is any one of these individuals. Through a
sequence of thought experiments, Elga in effect claims that he can derive the
credence function that should apply in such situations, thus justifying his
principle of indifference. Here we argue, using a Bayesian approach, that
Elga's reasoning is circular: in analyzing the third of his thought
experiments, he uses an assertion that is justifiable only if one assumes, from
the start, the principle of indifference that he is attempting to justify. We
agree with Elga that the assumption of equal credences is a very reasonable
principle, in the absence of any reason to assign unequal credences, but we do
not agree that the equality of credences can be so derived.
