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 critical relevance to SE.

A Basic View of Systems

The most basic ideas of a system whole can be found in both Western and Eastern philosophy. 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." (Bertalanffy 1968)

The parts of a system may be conceptual organizations of ideas 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.

Similar ideas of wholeness can be found in systems engineering literature. For example:

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).

The cohesive interactions between a set of parts, (Hitchins 2009, 59-63), allow us to identify a system boundary and defined what membership of the system means. 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 . The relationships between the various elements of an open system can be understood as a combination of the system 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 effects or outcomes 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. A stable system is one which returns to its original, or another, state following a disturbance in the environment.

The identification of a system and its boundary is ultimately the choice of the observer. This may be through observation and classification of sets of elements as systems, through an abstract conceptualisation of one or more possible boundaries and relationships in a given situation, or a mixture of both concrete and conceptual thinking. 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.

Many natural, social and man made things can be better understood by describing them as open systems. One of the reasons we find the idea of systems useful is that it is possible to identify shared concepts which apply to systems, and give useful insights into them, independently of the kinds of elements they are made up of. The ideas of structure, behavior and state are examples of such concepts. The identification of these shared system ideas, is the basis for Systems Thinking and their use in developing theories and approaches in a wide range of fields of study the system sciences.

Systems Engineering (SE), and a number of other Related Disciplines also use systems concepts, patterns and models in the creation of useful outcomes or things. The concept of a network of open systems created, 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 and provide a basis for effective problem solving.

Systems and System Elements

Many of the original ideas upon which GST, and other branches of system study, is based come from the study of systems in the biological and social sciences. Many natural 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. Such complex systems may exhibit specialization of elements, with elements taking on roles which contribute to the system purpose. Such roles might include self-regulation or control. Natural and social systems can be understood through an understanding of this wholeness, cohesion and specialization. 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.

Many man-made systems are designed as networks and hierarchies of related system elements to achieve similar resilience and desired behavior. Thus, it is often true that the elements of a system can themselves be considered as open systems. It can be useful to consider collections of related elements as both a system and a part of one or more other systems. A “holon” or system element was defined by Koestler as something which exists simultaneously a whole and as a part (Koestler 1967). Networks of related open systems at different levels of detail are the starting point for many applications of SE. The Systems Thinking knowledge area includes an overview of concepts, principles and patterns related to these types of system. There are two aspects of these systems which are of particular interest. Firstly, whole entities exhibit emergence, 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 complex behavior which is difficult to understand or predict. Both emergence and complexity can often best be dealt with not by looking for more detail, but by considering the wider open system relationships.

Bertalanffy (1968) divided open systems into nine real world types ranging from static structures and control mechanisms to socio-cultural systems. Other similar classification systems are discussed in the article Types of Systems. The following is a simple classification of system elements:

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

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 technical systems to fully realize their purpose. Systems which contain technical and either human or natural elements, are often called socio-technical systems. The behavior of such systems is determined both by the nature of the technical elements and by their ability to integrate with or deal with the variability of the natural and social systems around them.

System-of-Interest and Systems Context

Considering the relationships between a number of relevant system elements helps to fully explore problems and solutions. A given element may be included in several system views. However, we may also want to focus on a particular system as the primary focus of output of these activities. The idea of a System Context is used to define a System of Interest (SoI) and to identify the important relationships between its internal system elements, the wider system elements it works directly with, and the environmental conditions which influence it in some way. Context is a very important foundation of SE, since it combines the broad consideration of many related system elements with a focus on a particular socio-technical system deliverable.

The term socio-technical system is used by many in the systems community and may have meanings outside of that relevant to SE. Hence, we will define an engineered system as a system context in which a socio-technical system forms the primary focus across the whole of its life cycle , from initial problem formulation through to its final safe removal from use (INCOSE 2012). 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.

Systems engineering literature, standards and guides generally refer to their system-of-interest (SoI) as “the system” and their definitions of “a system” tend to characterize socio-technical (SoI) 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).

The International Council on Systems Engineering Handbook (INCOSE) (INCOSE 2012) generalizes this idea, defining 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." While this definition generally covers the SoI, we may also need to consider natural and social systems within the environment to fully understand an engineered system context. In SE and its related disciplines an engineered SoI is created and used by people for a purpose and may need to be considered across the whole of its life. While this is an important part of SE we can also consider the problem situation in which the engineered system sits, the social systems which developed, sustained and used them, and the commercial or public enterprises in which these all sit as systems (Martin 2004).

Hence, while many SE authors talk about systems and systems ideas they are often based on a particular world view which related to engineered systems and is focused on the creation of one or more systems of interest. It would also be useful to take a broader view of the context in which these SoI sit, and to consider through life relationships as part of that context. To help promote this the SEBoK will try to be more precise with its use of the word system, and will consider a wide set of system principles, pattern and models.


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.

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.

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.

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