This is in some sense a follow-up question to my previous question Why is it OK to keep the quadratic term in the small $\hbar$ approximation?. I understand how we can expand the action around a stationary point. This way we obtain a semiclassical expansion. I have read it in Coleman's Aspects of Symmetry that "If there are several stationary points, in general one has to sum over all of them".
I do not see why. I mean, we can always make the expansio around the stationary point of our choosing, isn't it? why do we have to sum all the expansions?