  In canonical quantization of gravity the wave function of the universe is CPT
invariant. Thus, if the quantum state of the universe contains a particular
history, than it must contain, with the same probability, the time-reversed
image of that history as well. In this work, we investigate the meaning of this
statement in the context of the conditional probability interpretation of Page
and Wootters. Accordingly, we show that a time-reversed history of the universe
is consistent with the Page and Wootters mechanism and we derive a
time-reversed Schrodinger equation for the evolution of the rest of the
universe. Since the same particular quantum state is acquired both in an
individual history with time running forward and in its time-reversed image
history, we demonstrate that conditioning the state of the universe on the
hands of the clock showing a specific time results in the probability of
finding the rest of the universe system in a particular state. In this
scenario, we conjecture that probability interpretation is an emergent property
of the quantum world.
