A generalised equivalence principle is put forward according to which
space-time symmetries and internal quantum symmetries are indistinguishable
before symmetry breaking. Based on this principle, a higher-dimensional
extension of Minkowski space is proposed and its properties examined. In this
scheme the structure of space-time is intrinsically quantum mechanical. It is
shown that the causal geometry of such a quantum space-time possesses a rich
hierarchical structure. The natural extension of the Poincare group to quantum
space-time is investigated. In particular, we prove that the symmetry group of
a quantum space-time is generated in general by a system of irreducible Killing
tensors. When the symmetries of a quantum space-time are spontaneously broken,
then the points of the quantum space-time can be interpreted as space-time
valued operators. The generic point of a quantum space-time in the broken
symmetry phase thus becomes a Minkowski space-time valued operator. Classical
space-time emerges as a map from quantum space-time to Minkowski space. It is
shown that the general such map satisfying appropriate causality-preserving
conditions ensuring linearity and Poincare invariance is necessarily a density
matrix
