  Some superstring theories have more than one effective low-energy limit,
corresponding to classical spacetimes with different dimensionalities. We argue
that all but the 3+1-dimensional one might correspond to ``dead worlds'',
devoid of observers, in which case all such ensemble theories would actually
predict that we should find ourselves inhabiting a 3+1-dimensional spacetime.
With more or less than one time-dimension, the partial differential equations
of nature would lack the hyperbolicity property that enables observers to make
predictions. In a space with more than three dimensions, there can be no
traditional atoms and perhaps no stable structures. A space with less than
three dimensions allows no gravitational force and may be too simple and barren
to contain observers.
