Difference-Engine (7.8) An agency whose actions tend to make the present state of affairs more like some goal or "desired state" whose description is represented in that agency. This idea was developed by Allen Newell, C. J. Shaw, and Herbert A. Simon into an important theory about human problem solving. See G. Ernst and Allen Newell, GPS, A Case Study in Generality and Problem Solving, Academic Press, 1969.