  This paper describes how the entire universe might be considered an
eigenstate determined by classical limiting conditions within it. This
description is in the context of an approach in which the path of each
relativistic particle in spacetime represents a fine-grained history for that
particle, and a path integral represents a coarse-grained history as a
superposition of paths meeting some criteria. Since spacetime paths are
parametrized by an invariant parameter, not time, histories based on such paths
do not evolve in time but are rather histories of all spacetime. Measurements
can then be represented by orthogonal states that correlate with specific
points in such coarse-grained histories, causing them to decohere, allowing a
consistent probability interpretation. This conception is applied here to the
analysis of the two slit experiment, scattering and, ultimately, the universe
as a whole. The decoherence of cosmological states of the universe then
provides the eigenstates from which our "real" universe can be selected by the
measurements carried out within it.
