The Noonday Argument: Fine-Graining, Indexicals, and the Nature of
  Copernican Reasoning
  Typicality arguments attempt to use the Copernican Principle to draw
conclusions about the cosmos and presently unknown conscious beings within it.
The most notorious is the Doomsday Argument, which purports to constrain
humanity's future from its current lifespan alone. These arguments rest on a
likelihood calculation that penalizes models in proportion to the number of
distinguishable observers. I argue that such reasoning leads to solipsism, the
belief that one is the only being in the world, and is therefore unacceptable.
Using variants of the "Sleeping Beauty" thought experiment as a guide, I
present a framework for evaluating observations in a large cosmos: Fine
Graining with Auxiliary Indexicals (FGAI). FGAI requires the construction of
specific models of physical outcomes and observations. Valid typicality
arguments then emerge from the combinatorial properties of third-person
physical microhypotheses. Indexical (observer-relative) facts do not directly
constrain physical theories. Instead they serve to weight different provisional
evaluations of credence. These weights define a probabilistic reference class
of locations. As indexical knowledge changes, the weights shift. I show that
the self-applied Doomsday Argument fails in FGAI, even though it can work for
an external observer. I also discuss how FGAI could handle observations in
large universes with Boltzmann brains.
