  We propose a new selection principle for distinguishing among possible vacua
that we call the "relaxation principle". The idea is that the universe will
naturally select among possible vacua through its cosmological evolution, and
the configuration with the biggest filling fraction is the likeliest. We apply
this idea to the question of the number of dimensions of space. We show that
under conventional (but higher-dimensional) FRW evolution, a universe filled
with equal numbers of branes and antibranes will naturally come to be dominated
by 3-branes and 7-branes. We show why this might help explain the number of
dimensions that are experienced in our visible universe.
