acts upstream of c acts upstream of p if and only if c enables some f that is involved in p' and p' occurs chronologically before p, is not part of p, and affects the execution of p. c is a material entity and f, p, p' are processes. A faulty traffic light (material entity) whose malfunctioning (a process) is causally upstream of a traffic collision (a process): the traffic light acts upstream of the collision. acts upstream of or within enables causally upstream of