  In the model of holographic dark energy, there is a notorious problem of
circular reasoning between the introduction of future event horizon and the
accelerating expansion of the universe. We examine the problem after dividing
into two parts, the causality problem of the equation of motion and the
circular logic on the use of the future event horizon. We specify and isolate
the root of the problem from causal equation of motion as a boundary condition,
which can be determined from the initial data of the universe. We show that
there is no violation of causality if it is defined appropriately and the
circular logic problem can be reduced to an initial value problem.
