  In the path integral formulation of quantum mechanics, the phase factor
Exp[iS(x[t])] is associated with every path x[t]. Summing this factor over all
paths yields Feynman's propagator as a sum-over-paths. In the original
formulation, the complex phase was a mathematical device invoked to extract
wave behaviour in a particle framework. In this paper we show that the phase
itself can have a physical origin in time reversal, and that the propagator can
be drawn by a single deterministic path.
