Introduction to Systems Engineering Fundamentals

This article forms part of the Systems Thinking Knowledge Area. It provides various perspectives on systems, including definitions, scope, and context. The basic definitions in this article are further expanded and discussed in Types of Systems and What is Systems Thinking?.

This article asks the following question: how can the many connotations of the word system in everyday use be turned into a theoretical framework capable of supporting the application of systems thinking in such a way that it can be usefully applied across all potential application domains and disciplines?

Systems Science View

The most basic ideas of a system whole can be traced back to the thinking of Aristotle and culminate in the works of the philosopher Hegel (M’Pherson 1974). Hegel’s view of holism states that an individual as part of an organic whole is fully realized only through his relationship to the whole. Humans have used this idea to help explain the relationships between abstract ideas by describing them as a system of related concepts, rules, or ideas. Some examples of such systems are the natural number system and political systems. The SEBoK uses the notion of a “system of ideas” to help present and explain system and systems engineering (SE) knowledge.

A system is a combination of elements with some relationship(s) which can be best understood by considering them as a whole. The generally agreed upon systems science definition of a system considers a set of related elements in an environment which contains related systems and conditions:

• A closed system has no relationship(s) with its environment.
• An open system shares both inputs and outputs with its environment across the boundary.

System elements may be conceptual organizations of ideals in symbolic form or real objects; e.g., people, data, physical artifacts, etc.

• Abstract systems contain only conceptual elements.
• Concrete systems contain at least two elements which are objects.

Many natural systems and social systems are formed through the inherent cohesion between elements, which leads them to form stable structures and hold those structures when disturbed. We can observe a process in nature where simple systems develop into more complex systems over time (Simons 1962).

Laszlo summarizes the properties which prompt considering a set of related elements as a whole (Laszlo 1972):

1. Systemic state (or wholeness): A property of the system elements and how they are related in the system structure which leads them to create a cohesive whole.
2. Adaptive self-regulation: All systems will tend to return to a defined steady state in response to external stimulus.
3. Adaptive self-organization: Some systems not only return to a previous steady state, but also reorganize to create new steady states which are more resistant to change.
4. Holon: Systems displaying characteristics two and three will tend to develop increasing hierarchical structures.

Thus, a collection of elements which tend to group together will form themselves into coherent wholes. 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. 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.

Natural and social systems can be understood and managed through an understanding of this wholeness. engineered systems can be deliberately created to take advantage of this holism .

General System Theory (Bertalanffy 1968) considers the similarities between systems from different domains as a set of common system principles and concepts. The following system concepts, derived from General System Theory, relate directly to the definition of a system:

1. Wholeness: All systems are formed from groups of related elements into a whole with an observable shared identity in an environment. Many systems will form very stable relationships and tend to maintain their wholeness in the face of external change.
2. Behavior: All systems exhibit behaviors resulting from the interaction between elements. This may range from simple stimulus responses to conscious goal seeking actions.
3. Control: All systems have regulation and control mechanisms to guide their behaviors. For some systems, control may be a consequence of the relationships between its elements, while others may exhibit control mechanisms in a way guided by an identified purpose.
4. Hierarchy: All systems form hierarchical structures and will exhibit additional behaviors that emerge within the hierarchy due to interaction between system elements.
5. Complexity: At some levels of a hierarchy, systems are sufficiently complex that the ability of an observer to understand, interact with, or change them may be reduced.

While abstract systems are a useful way of expressing complex ideas and have value in systems science, it is the study of concrete systems which forms the majority of the systems knowledge of direct relevance to SE.

A concrete system contains a collection of elements and relationships which form coherent wholes. Systems exploit this cohesion to sustain themselves in the face of threats or environmental pressures, as well as to produce behaviors not possible by the elements alone. Some systems are created by people for specific reasons and will need to not only exist and survive, but also achieve necessary outcomes. At some point, the nature of the relationships between elements leads to complexity not associated with the elements themselves; this complexity can only be dealt with by considering the system as a whole.

The joining together of the various elements of a system can be related to either the structure of a system or to the behavior of a system. The structure of a system is its static existence, namely its elements and their relationships, whereas system behavior refers to the effect produced when an instance of the system is in operation. The actual behavior produced in operation 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).

The system context describes the system in its environment. The system context includes the boundaries of the system of interest and the relationship of that system of interest to the environment in which it exists.

Understanding these system concepts and associated principles forms the basis of systems thinking. An expanded system of these system concepts are described in more detail in System Concepts.

System Definitions – a Discussion

How is a system defined in the SE literature? Systems engineers generally refer to their system of interest (soi) as “the system,” and their definitions of a “system” tend to characterize engineered systems . Two examples follow:

• “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 INCOSE Handbook (2011) 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 don’t 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. This has led to more general definitions of a system following the systems science approach. For example, Aslaksen (2004) says a system consists of the following three related sets:

• a set of elements;
• a set of internal interactions between the elements; and
• a set of external interactions between one or more elements and the external world (i.e., interactions that can be observed from outside of the system).

This definition of a system enables people to reason about numerous classes of dynamic systems that involve engineered, social, and natural systems.

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.

Natural systems and social systems often form part of the environment in which engineered systems need to exist. For a natural or social system, simply continuing to exist, and when appropriate, to adapt and grow, is sufficient. Man-made, engineered, and sociotechnical systems are created with a defined purpose (Hitchins 2009). Thus, these systems must not only be able to exist within their environment, but also do what is necessary to achieve their purpose.

System of Interest

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 complexity 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 when 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 system of interest (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. There are a number of more detailed system concepts which the systems approach must also consider, such as static or dynamic, deterministic or non-deterministic, chaotic or homeostatic, complexity and adaptation, feedback and control, and more.

All applications of a systems approach (and hence of SE) are applied to an engineered systems context, and not to an individual system.

References

Works Cited

Alaksen, E. 2004. "A Critical Examination of the Foundations of Systems Engineering Tutorial". Paper presented at 14th Annual International Council on Systems Engineering (INCOSE) International Symposium, Toulouse, France, 20-24 June 2004.

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.

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

INCOSE. 2011. INCOSE Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.

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.

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

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.

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. 2011. INCOSE Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), INCOSE-TP-2003-002-03.2.1.

Additional References

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

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