  The observable universe is necessarily hospitable for life. There are
indications, however, that the laws of physics and cosmological parameters need
not take the form and values observed, and if they were slightly different life
could not exist. A common approach to this fine tuning problem is to propose a
cosmos with an ensemble of domains, mostly inhospitable for life. A Bayesian
method is used to show that this hypothesis is more credible than a homogeneous
fine tuned universe. This conclusion is straightforward for a finite ensemble,
but can be extended to an infinite ensemble by applying a formulation of the
Principle of Mediocrity.
