  String theory requires additional degrees of freedom to maintain world-sheet
reparameterisation invariance at the quantum level. These are often interpreted
as extra dimensions, beyond the 4 space-time. I discuss a class of
quasi-realistic string models in which all the untwisted geometrical moduli are
projected out by GSO projections. In these models the extra dimensions are
fictitious, and do not correspond to physical dimensions in a low energy
effective field theory. This raises the possibility that extra dimensions are
fictitious in phenomenologically viable string vacua. I propose that
self-duality in the gravitational quantum phase-space provides the criteria for
the string vacuum selection.
