System wholes have behaviors and properties arising from the organization of their elements and their relationships, which only become apparent when the system is placed in different environments. Questions that arise from this definition include: What kinds of systems exhibit different kinds of emergence and under what conditions? Can emergence be predicted, and is it beneficial or detrimental to a system? How do we deal with emergence in the development and use of engineered systems? Can it be planned for? How? There are many varied and occasionally conflicting views on emergence. This topic presents the prevailing views and provides references for others.
Overview of Emergence
As defined by Checkland, emergence is “the principle that entities exhibit properties which are meaningful only when attributed to the whole, not to its parts.” (Checkland 1999, p. 314). Emergent system behavior can be viewed as a consequence of the interactions and relationships between system elements rather than the behavior of individual elements. It emerges from a combination of the behavior and properties of the system elements and the systems structure or allowable interactions between the elements, and may be triggered or influenced by a stimulus from the systems environment.
Hitchins also notes that technological systems exhibit emergence. We can observe a number of levels of outcome which arise from interaction between elements in an engineered system context. At a simple level, some system outcomes or attributes have a fairly simple and well defined mapping to their elements; for example, center of gravity or top speed of a vehicle result from a combination of element properties and how they are combined. Other behaviors can be associated with these simple outcomes, but their value emerges in complex and less predictable ways across a system.
Emergence can always be observed at the highest level of system. However, Hitchins (2007, p.7) also points out that to the extent that the systems elements themselves can be considered as systems, they also exhibit emergence. Page (2009) refers to emergence as a “macro-level property.” Ryan (2007) contends that emergence is coupled to scope rather than system hierarchical levels. In Ryan’s terms, scope has to do with spatial dimensions (how system elements are related to each other) rather than hierarchical levels.
Types of Emergence
A variety of definitions of types of emergence exists. See Emmeche et al.(1997), Chroust (2003) and O’Connor and Wong (2006) for specific details of some of the variants. Page (2009) describes three types of emergence: "simple", "weak", and "strong".
According to Page, simple emergence is generated by the combination of element properties and relationships and occurs in non-complex or “ordered” systems (see Complexity) (2009). To achieve the emergent property of “controlled flight” we cannot consider only the wings, or the control system, or the propulsion system.
Page describes weak emergence as expected emergence which is desired (or at least allowed for) in the system structure (2009). However, since weak emergence is a product of a complex system, the actual level of emergence cannot be predicted just from knowledge of the characteristics of the individual system components.
The term strong emergence is used to describe unexpected emergence; that is, emergence not observed until the system is simulated or tested or, more alarmingly, until the system encounters in operation a situation that was not anticipated during design and development.
A type of system particularly subject to strong emergence is the System of Systems (SoS) (glossary). The reason for this is that the SoS, by definition, is composed of different systems that were designed to operate independently.
When these systems are operated together, the interaction among the parts of the system is likely to result in unexpected emergence. Chaotic or truly unpredictable emergence is likely for this class of systems.
Emergent Properties
Emergent properties can be defined as follows: “A property of a complex system is said to be ‘emergent’ [in the case when], although it arises out of the properties and relations characterizing its simpler constituents, it is neither predictable from, nor reducible to, these lower-level characteristics” (Honderich 1995, pg 224).
All systems can have emergent properties which may or may not be predictable or amenable to modeling, as discussed above. Much of the literature on complexity includes emergence as a defining characteristic of complex systems.