What is a System?

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This article forms part of the Systems Fundamentals knowledge area (KA). It provides various perspectives on systems, including definitions, scope, and context. The basic definitions in this article are further expanded and discussed in the articles Types of Systems and What is Systems Thinking?.

This article provides a guide to some of the basic concepts of systems developed by systems science and discusses how these relate to the definitions to be found in systems engineering (SE) literature. The concept of an engineered system is introduced as the system context of most relevance to SE.


A Basic Systems Science View

The most basic ideas of a system whole can be traced back to the thinking of Greek philosophers such as Aristotle and Plato. Many philosophers have considered notions of holism, that ideas, people or things must be considered in relation to the things around them to be fully understood (M’Pherson 1974).

One influential systems science definition of a system comes from general system theory (GST):

"A System is a set of elements in interaction." (von Bertalanffy 1968)

The elements of a system may be conceptual organizations of ideals in symbolic form or real objects. GST considers abstract systems to contain only conceptual elements and concrete systems to contain at least two elements that are real objects, e.g. people, information, software and physical artifacts, etc.

GST starts with the notion of a system boundary defined by those relationships which relate to membership of the system. The setting of a boundary and hence the identification of a system is ultimately the choice of the observer. This underlines the fact that any particular identification of a system is a human construct used to help make better sense of a set of things and to share that understanding with others if needed.

For closed systems all aspects of the system exist within this boundary. This idea is useful for abstract systems and for some theoretical system descriptions. The boundary of an open systems defines those elements and relationships which can be considered part of the system and those which describe the interactions across the boundary between system elements and elements in the environment .

Systems thinking and systems science and some systems approaches make use of abstract closed systems of ideas to define and organize concepts. The concept of a network of open systems sustained and used to achieve a purpose within one or more environments is a powerful model that can be used to understand many complex real world situations; additionally, it can provide a basis for effective problem solving.

Open Systems

The relationships between the various elements of an open system can be related to a combination of the system's structure and behavior. The structure of a system describes a set of system elements and the allowable relationships between them. System behavior refers to the effect produced when an instance of the system interacts with its environment. An allowable configuration of the relationships between elements is referred to as a system state and the set of allowable configurations as its state space.

The following is a simple classification of system elements:

  • Natural elements, objects or concepts which exist outside of any practical human control. Examples: the real number system, the solar system, planetary atmosphere circulation systems.
  • Human elements, either abstract human types or social constructs, or concrete individuals or social groups.
  • Technological elements, man-made artifacts or constructs; including physical hardware, software and information.

A system may be made up of a network of system elements and relationships at a single level of detail or scale. However, many systems evolve or are designed as hierarchies of related systems. Thus, it is often true that the elements of a system can themselves be considered as open systems. A “holon” was defined by Koestler as something which exists simultaneously a whole and as a part (Koestler 1967).

Laszlo (1972) summarizes the open system property of holism (or systemic state) as a property of the system elements and how they are related in the system structure that leads them to create a cohesive whole. Open systems can persist when the relationships between the elements reach a balance which remains stable within its environment. Laszlo describes three kinds of system response to environmental disturbance:

  • Adaptive Self-Regulation - Systems will tend to return to their previous state in response to external stimulus.
  • Adaptive Self-Organization - Some systems not only return to a previous state, but also reorganize to create new stable states which are more resistant to change.
  • Holonic - Systems displaying characteristics one and two will tend to develop increasingly complex (hierarchical) structures.

The observed behavior of a system in its environment leads to the fundamental property of emergence. Whole entities exhibit properties which are meaningful only when attributed to the whole, not to its parts… (Checkland 1999). At some point, the nature of the relationships between elements within and across boundaries in a hierarchy of systems may lead to behavior which is difficult to understand or predict. This system complexity can only be dealt with by considering the systems as a collective whole.

Open Systems Domains

Bertalanffy (1968) further divided open systems into nine types ranging from static structures and control mechanisms to socio-cultural systems. Other similar classification systems are discussed in the article Types of Systems.

In the SEBoK, three related open system domains are considered:

  • A natural system is one whose elements are wholly natural.
  • A social system includes only humans as elements.
  • An engineering system is a man-made aggregation which may contain physical, informational, human, natural and social elements; it is normally created for the benefit of people.

These three types overlap to cover the full scope of real-world open, concrete systems.

Figure 1. System Boundaries of Engineered Systems, Social Systems, and Natural Systems. (SEBoK Original)

Natural systems are real world phenomena to which systems thinking is applied to help better understand what those systems do and how they do it. A truly natural system would be one that can be observed and reasoned about, but over which people cannot exercise direct control, such as the solar system.

Social systems are purely human in nature, such as legislatures, conservation foundations, and the United Nations Security Council. These systems are human artifacts created to help people gain some kind of control over, or protection from, the natural world.

While the above distinctions can be made as an abstract classification, in reality, these are not hard and fast boundaries between these types of systems: e.g., social systems are operated by, developed by, and also contain natural systems and social systems depend on engineered systems to fully realize their purpose and thus will form part of one or more engineered systems contexts.

Engineered systems may be purely technical systems, such as bridges, electric autos, and power generators. Engineered systems which contain technical and either human or natural elements, such as water and power management, safety governance systems, dams and flood control systems, water and power safety assurance systems are often called sociotechnical systems . The behavior of such systems is determined both by the nature of the engineered elements and by their ability to integrate with or deal with the variability of the natural and social systems around them. The ultimate success of any engineered system is thus measured by its ability to contribute to the success of relevant sociotechnical system contexts.

Many of the original ideas upon which GST is based come from the study of systems in the biological and social sciences. Many natural systems and social systems are formed through the inherent cohesion between elements. Once formed, they will tend to stay in this structure, as well as combine and evolve further into more complex stable states to exploit this cohesion in order to sustain themselves in the face of threats or environmental pressures, as well as to produce other behaviors not possible from simpler combinations of elements. Natural and social systems can be understood through an understanding of this wholeness and cohesion. They can also be guided towards the development of behaviors which not only enhance their basic survival, but also fulfill other goals or benefit to them or the systems around them. The Architecture of Complexity (Simon 1962) has shown that systems which evolve via a series of stable “hierarchical intermediate forms” will be more successful and adapt more quickly to environmental change.

Some systems are created by people for specific reasons and will need to not only exist and survive, but also achieve necessary outcomes. Engineered systems can be deliberately created to take advantage of system properties such as holism and stability, but must also consider system challenges such as complexity and emergence.

There are a number of more detailed system concepts which must also be consider, such as static or dynamic, deterministic or non-deterministic, chaotic or homeostatic, complexity and adaptation, feedback and control, and more. Understanding these system concepts and associated principles forms the basis of systems thinking. An expanded discussion of these concepts is given in the article Concepts of Systems Thinking.

System Definitions – A Discussion

How is system defined in the SE literature?

Fundamental properties of a system described in the SE literature include togetherness, structure, behavior, and emergence. These properties provide one perspective on what a system is. We believe that the essence of a system is 'togetherness', the drawing together of various parts and the relationships they form in order to produce a new whole… (Boardman and Sauser 2008). Hitchins (2009, 59-63) refers to this systems property as cohesion.

Systems engineers generally refer to their system-of-interest (SoI) as “the system” and their definitions of “a system” tend to characterize technology focused systems with a defined purpose, e.g.

  • “A system is a value-delivering object” (Dori 2002).
  • “A system is an array of components designed to accomplish a particular objective according to plan” (Johnson, Kast, and Rosenzweig 1963).
  • “A system is defined as a set of concepts and/or elements used to satisfy a need or requirement" (Miles 1973).

Thus SE definitions refer to engineered systems, containing combinations of technology and people created to achieve a goal or purpose of value to one or more stakeholders (Hitchins 2009).

The International Council on Systems Engineering Handbook (INCOSE) 2012) generalizes this idea of an engineered system as “an interacting combination of elements to accomplish a defined objective. These include hardware, software, firmware, people, information, techniques, facilities, services, and other support elements.”

However, engineered systems often find that their environment includes natural systems that do not follow the definitions of a “system” above in that they have not been defined to satisfy a requirement or come into being to satisfy a defined objective. These include such systems as the solar system if one’s engineered system is an interplanetary spacecraft.


As can be seen from the discussion above, most attempts to define the term “system” in SE either include assumptions about the system domain being considered, or are attempts to take a systems science view which risk becoming too abstract to be of practical use. A clear distinction is needed between defining "the system" to which a systems approach is applied and defining "systems" as an abstract idea which can be used to help understand complex situations.

The concept of a system helps make sense of the glossary of the real world. This is done either by creating an abstract system to help explain complex situations, such as the real number system, by creating a standardized approach to common problems, such as the Dewey Decimal System, or by agreeing on a model of a new situation to allow further exploration, such as a scientific theory or conceptual system design. People use systems to make sense of complexity in an individual way and then they work together to solve problems.

In the systems approach, a number of relevant systems may be considered to fully explore problems and solutions and a given element may be included in several system views. Thus, it is less important that “the system” can be defined than it is that combinations of systems can be used to help achieve engineering or management tasks.

The idea of a system context is used to define a SoI and to identify the important relationships between it, the systems it works directly with, and the systems which influence it in some way. This engineered system context relates to the systems science ideas of an open, concrete system, although such a system may include abstract system elements.

An engineered system is created and used by people for a purpose and may need to be considered across the whole of its life, from initial problem formulation through to its final safe removal from use (INCOSE 2012). A systems context view can be taken not only as the engineered systems we create to fulfill a purpose, but also the problem situation in which they sit, the systems which developed, sustained and used them, and the commercial or public enterprises in which these all sit (Martin 2004).


Works Cited

Bertalanffy, L. von. 1968. General System Theory: Foundations, Development, Applications, rev. ed. New York: Braziller.

Boardman, J. and B. Sauser. 2008. Systems Thinking: Coping with 21st Century Problems. Boca Raton, FL, USA: Taylor & Francis.

Checkland, P. 1999. Systems Thinking, Systems Practice. New York, NY, USA: Wiley and Sons, Inc.

Dori, D. 2002. Object-Process Methodology – A Holistic Systems Paradigm. Verlag, Berlin, Heidelberg, New York: Springer.

Hitchins, D. 2009. “What Are the General Principles Applicable to Systems?” INCOSE Insight, 12(4): 59-63.

INCOSE. 2012. Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.2. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.2.

Johnson, R.A., F.W. Kast, and J.E. Rosenzweig. 1963. The Theory and Management of Systems. New York, NY, USA: McGraw-Hill Book Company.

Koestler, A. 1990. The Ghost in the Machine, 1990 reprint ed. Penguin Group.

Laszlo, E., ed. 1972. The Relevance of General Systems Theory: Papers Presented to Ludwig von Bertalanffy on His Seventieth Birthday. New York, NY, USA: George Brazillier.

Martin, J, 2004. "The Seven Samurai of Systems Engineering: Dealing with the Complexity of 7 Interrelated Systems". Proceedings of the 14th Annual International Council on Systems Engineering International Symposium, 20-24 June, 2004, Toulouse, France.

Miles, R.F. (ed). 1973. System Concepts. New York, NY, USA: Wiley and Sons, Inc.

M’Pherson, P.K. 1974. "A perspective on systems science and systems philosophy". Futures. 6(3):219-39.

Simon, H.A. 1962. "The Architecture of Complexity." Proceedings of the American Philosophical Society. 106(6) (Dec. 12, 1962): 467-482.

Primary References

Bertalanffy, L., von. 1968. General System Theory: Foundations, Development, Applications, rev. ed. New York, NY, USA: Braziller.

INCOSE. 2012. Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.2. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.2.

Additional References

Hybertson, Duane. 2009. Model-oriented Systems Engineering Science: A Unifying Framework for Traditional and Complex Systems. Boca Raton, FL, USA: CRC Press.

Hubka, Vladimir, and W. E. Eder. 1988. Theory of Technical Systems: A Total Concept Theory for Engineering Design. Berlin: Springer-Verlag.

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