with a closed system (universe) , splits it into a “relevant" part (system) and an “irrelevant" (environment) part according to the specific physical conditions of the problem (for example, the split between light and heavy modes or slow and fast variables), and follows the behavior only of the relevant variables (now comprising an open system) by integrating out the irrelevant variables. (See Hu 1993a and 1993b for a discussion of the meaningfulness of such splits in the general physical and cosmological context respectively). Thus the coarse-grained effective action generalizes the background field/ fluctuation-field splitting usually assumed when carrying out a quantum loop expansion in the ordinary effective action calculation to that between a system field and an environment field. It takes into account the averaged effect of the environment variables on the system and produces an effective equation of motion for the system variables. This method was used by Hu and Zhang (1990) (see also Hu 1991) to study the effect of coarse-graining the higher modes in stochastic inflation, and by Sinha and Hu (1991) to study the validity of minisuperspace approximations in quantum cosmology. The last step in completing this tour is the recognition by Hu, Paz and Zhang 