Given a current model of the world and a current goal, STRIPS attempts to prove the goal in the world model by resolution. If the goal is satisfied, the search can move on to the next goal, or succeed if the main goal is satisfied.
If the current goal cannot be satisfied in the current world model, STRIPS takes clauses derived from the goal as new subgoals, or as a difference between the world model and the goal (as in GPS).
Example: Suppose the negated goal is ¬ At(Box1, x) &or ¬ At(Box2, x) and the world model has At(Box1, b). Resolution gives the new subgoal At(Box2, b).
An operator relevant to reducing the difference can be found by looking for operators whose Add Lists unify with parts of the subgoal. After selecting an operator, the preconditions of the operator become additional subgoals.