  A general condition for the self-consistency of a semiclassical approximation
to a given system is suggested. It is based on the eigenvalue distribution of
the relevant Hessian evaluated at the streamline configurations (configurations
that almost satisfy the classical equations of motion). The semiclassical
approximation is consistent when there exists a gap that separates small and
large eigenvalues and the spreading among the small eigenvalues is much smaller
than the gap. The idea is illustrated in the case of the double-well potential
problem in quantum mechanics. The feasibility of the present idea to test
instanton models of QCD vacuum is also briefly discussed.
