  We define the idea of {\it real path quantum theory}, a realist
generalisation of quantum theory in which it is postulated that the
configuration space path actually followed by a closed quantum system is
probabilistically chosen. This is done a postulate defining probabilities for
paths, which we propose are determined by an expression involving path
amplitudes and a distance function that quantifies path separation. We suggest
a possible form for a path probability postulate and explore possible choices
of distance function, including choices suitable for Lorentz or generally
covariant versions of real path quantum theory. We set out toy models of
quantum interferometry and show that in these models the probability postulate
and specific distance functions do indeed give a physically sensible path
ontology. These functions can be chosen so as to predict quantum interference
for interference of microscopic quantum systems and the failure of interference
for macroscopic quantum systems. More generally, they predict interference when
the beams are close, and its failure when they are far apart, as determined by
the distance function. If taken seriously in its present relatively
unconstrained form, real path quantum theory thus motivates experimental tests
of quantum interference in all unexplored regimes defined by potentially
physically interesting parameters, including the mass of the beam object, the
beam separation distance, the beam separation time, and many others. We discuss
open questions raised by these ideas.
