  In cosmology, we would like to explain our observations and predict future
observations from theories of the entire universe. Such cosmological theories
make ontological assumptions of what entities exist and what their properties
and relationships are. One must also make epistemological assumptions or
metatheories of how one can test cosmological theories. Here I shall propose a
Bayesian analysis in which the likelihood of a complete theory is given by the
normalized measure it assigns to the observation used to test the theory. In
this context, a discussion is given of the trade-off between prior
probabilities and likelihoods, of the measure problem of cosmology, of the
death of Born's rule, of the Boltzmann brain problem, of whether there is a
better principle for prior probabilities than mathematical simplicity, and of
an Optimal Argument for the Existence of God.
