Emergence and Hierarchy

The foundation of systems theory rests on two pairs of ideas (1) emergence and hierarchy and (2) communication and control [36], 

Reliability is a component property. Conclusions can be reached about the reliability of a valve in isolation, where reliability is defined as the probability that the behavior of the valve will satisfy its specification over time and under given conditions. 

Safety, on the other hand, is clearly an emergent property of systems Safety can be determined only in the context of the whole. Determining whether a plant is acceptably safe is not possible, for example, by examining a single valve in the plant. In fact, statements about the safety of the valve without information about the context in which that valve is used are meaningless. Safety is determined by the relationship between the valve and the other plant components. As another example, pilot procedures to execute a landing might be safe in one aircraft or in one set of circumstances but unsafe in another. 

Although they are often confused, reliability and safety are different properties. The pilots may reliably execute the landing procedures on a plane or at an airport in which those procedures are unsafe. A gun when discharged out on a desert with no other humans or animals for hundreds of miles may be both safe and reliable. When discharged in a crowded mall, the reliability will not have changed, but the safety most assuredly has. 

Because safety is an emergent property, it is not possible to take a single system component, like a software module or a single human action, in isolation and assess its safety. A component that is perfectly safe in one system or in one environment may not be when used in another. 

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