Evaluation strategies, for Darden, involve constraint-based reasoning to limn the contours of the space of possible mechanisms for a given phenomenon. Often scientists reason about how a mechanism works by building off basic findings concerning the spatial and temporal organization of its parts. Harvey, for example, reasoned his way to the circulation of the blood by considering the locations of the valves of the veins and their orientation with respect to the heart. These organizational constraints, and many others, combined to narrow the space of possible mechanisms to a small region containing a model in which the blood completes a circuit of the body (Craver and Darden 2013).