  Different aspects of relativity, mainly in a canonical formulation, relevant
for the question "Is spacetime nothing more than a mathematical space (which
describes the evolution in time of the ordinary three-dimensional world) or is
it a mathematical model of a real four-dimensional world with time entirely
given as the fourth dimension?" are presented. The availability as well as
clarity of the arguments depend on which framework is being used, for which
currently special relativity, general relativity and some schemes of quantum
gravity are available. Canonical gravity provides means to analyze the field
equations as well as observable quantities, the latter even in coordinate
independent form. This allows a unique perspective on the question of
dimensionality since the space-time manifold does not play a prominent role.
After re-introducing a Minkowski background into the formalism, one can see how
distinguished coordinates of special relativity arise, where also the nature of
time is different from that in the general perspective. Just as it is of
advantage to extend special to general relativity, general relativity itself
has to be extended to some theory of quantum gravity. This suggests that a
final answer has to await a thorough formulation and understanding of a
fundamental theory of space-time. Nevertheless, we argue that current insights
into quantum gravity do not change the picture of the role of time obtained
from general relativity.
