ABSTRACT
When we add interactions, we can no longer solve the resulting interacting quantum theory (except for a few special cases), so we must be satisfied with an approximate solution. The approximate solution is usually a perturbative expansion of some sort, where the lowest order term is the free quantum theory or the interacting classical theory. Our confidence in the validity of the series is based on what we believe to be the identification of a "small" expansion parameter, usually a coupling constant or h, and much of the way we interpret the results relies on the interpretation of the lowest order term.
