  It is shown by very simple arguments that the observed 3+1 dimensionality of
spacetime may be understood on the basis of four fundamental principles of
physics namely, Causality, General Covariance, Gauge Invariance and
Renormalizability. This approach not therefore only fixes the spacetime
dimensionality at 3+1, but also imposes the compactification of extra
dimensions in higher dimensional theories based on these principles such as
string theory as an internal consistency requirement rather than an external
imposition.
