This chapter proposes to examine two aspects of the relationship between individual elements and the systems of which they are the constituent parts. The aspects are safety of the parts versus the safety of the whole, and detailed specification about how each part is to be built and used versus measures of performance for the plant as a whole or for major segments of it. The Nuclear Regulatory Commission (NRC) is responsible for licensing and inspecting civilian users of nuclear materials. NRC design review groups are very much aware of this type of system interaction: "Water is, of course, required for fire fighting systems," an official observed. This water is distributed by pipes running in places where there is electrical equipment important to safety. The Three Mile Island accident provides several examples of conflicting efforts, each justified in the name of safety, and each, taken alone, probably productive of safety.