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First published online 08:28 BST 2nd April 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 09:00 GMT 15th March 2011
Earlier versions of this material appeared in Smith (1997; Chapter 4). It is simplified here and supported with hyperlinks.
Although this paper is reasonably self-contained, it is best read as a glossary supporting our larger paper on Control Theory. To go directly to the superordinate content file, click here, and to see the author's homepage, click here.
1 - Systems Theory Background
Control Theory concepts have a distinct history outside the realm of technology, being frequently encountered in social sciences such as politics, economics, and management. Mayr (1971), for example, makes a good case for dating the concepts of feedback and self-regulation in economic theory to David Hume's essay "On the Balance of Trade" (Hume, 1752), and they are also easy to see in the discussion of defective self-regulation in Thomas Malthus' (1798) "Essay on the Principle of Population". Formal theories did not appear, however, until the early twentieth century. Lotka (1925, cited in Kramer and de Smit, 1977) was the first to speak of "open systems" - systems which interacted with, and responded to, their environment - and he was closely followed by Cannon (1927), who used the concept of homeostasis to explain a whole variety of biological phenomena, and by Von Bertalanffy (1932), who further developed the vocabulary and popularised the title "system theory" [nowadays more frequently seen as "systems theory"].
What all such explanations have in common is the fact that they take all-purpose rules derived from a wide range of exemplar systems and apply these as abstract principles to other systems, including those about which little is currently known. System theory is thus the science of the global explanatory principle. It is the science of "the isomorphism of natural laws" (Von Bertalanffy, 1950, p138), and that makes it yet another aid to determining the computational principles of poorly understood systems. Here is an entry-level vocabulary:
2 - Systems Theory Glossary
Attributes: The qualities of an entity, such as its size and weight, etc. The defining characteristics of a thing. Anything, indeed, which assists an entity's detection and identification.
Cannon (1927): A classic paper describing how biological systems make extensive use of negative feedback loops to control various physiological indices (eg. blood sugar, blood oxygen, blood pressure, etc.), enabling each index to adjust itself automatically when faced with fluctuating demand, usually by invoking a process known as homeostasis. [See now General Living System Theory. For more on feedback loops, see our e-paper on "Basics of Cybernetics".]
Closed System: A system which has no interaction with its system environment. [Compare open system.]
Entity: Entities are "the elements or parts of a system" (Kramer and de Smit, 1977, p14). They are "to a significant degree discontinuous with the environment that contains them" (Salthe, 1985, p23), their individual qualities are known as attributes, and they influence each other through relations.
Functionality: The overall benefit accruing from, or provided by, a system. That which a system does or exists to provide, and therefore the sum total of products or services specified in that system's Requirements Specification, if one exists.
General Living System Theory (GLST): [See firstly General System Theory.] A subset of General System Theory developed by the pioneer systems scientist James Grier Miller (1916-), who, in Miller (1978), argued that no less than 19 critical subsystems - most of them essentially homeostatic - are necessary for the survival of a complex living system.
General System Theory (GST): An attempt by Ludwig Von Bertalanffy (1901-1972) to develop a mathematically exact theory in the non-physical sciences (eg. Von Bertalanffy, 1956). A formal mathematical model of the interactions between the subsystems making up a larger system. [See also General Living System Theory.]
Homeostasis: Cannon's (1927) term for "the ability of an organism to maintain a constant internal environment (i.e., body temporature, fluid content, etc.) through regulatory mechanisms that compensate for a changing external environment" (Hyperdictionary, 2004). Alternatively, "a term used in systems thinking to describe the action of negative feedback processes in maintaining the system at a constant equilibrium state" (ibid.).
Interface: [See firstly relation.] A point of informational or substantive exchange between the subsystems making up a system, or, via the system environment, between systems. If information is interfaced, then the process involves the cross-setting of attributes from one end of the interface pathway to the other (and, usually, back again). If commodity is interfaced, then the process involves the physical transfer of entity.
Open System: A system which can influence or be influenced by external entities, whether free-standing, or themselves part of other systems. One which interacts with its system environment rather than being isolated from it (implying that living systems are typically open systems). [Compare closed system.]
Relation: [Sometimes "relationship".] "The way in which two or more entities are dependent on each other" (Kramer and de Smit, 1977, p15). This being so, then a relation (or network of relations) may be suspected "if a change in a property of one entity results in a change in a property of another entity" (Kramer and de Smit, 1977, p16; italics original). [Note that a relation is not automatically an interface. An interface is a physical pathway between two subsystems, and therefore implements what might originally have been a multi-nodal web of entities and relations. A relation, by contrast, is (a) a logical association rather than a physical one, and (b) exists between entities rather than between subsystems.]
Requirements Specification: [Alternatively, Statement of Requirements (SOR).] For formally engineered man-made systems, the Requirements Specification is the document which states the functionality required of the system. It is produced during the early stages of the broader process of systems engineering.
Subsystem: [See firstly system.] "A functional component of a larger system which fulfils the conditions of a system in itself, but which also plays a role in the operation of a larger system" (Young, 1964, cited in Kramer and de Smit, 1977, p26).
System: "A set of interrelated entities of which no subset is unrelated to any other subset" (Kramer and de Smit, 1977, p14). Alternatively, a system "is composed of interrelated parts but can be perceived as a whole [and], from a systems perspective, this whole is more than the sum total of the parts of the system." (Verstraete, 1998/2003 online).
System Boundary: The notional dividing line between a system and its system environment.
System Environment: [See firstly system boundary.] "That set of entities outside the system, the state of which set is affected by the system or which affect the state of the system itself" (Kramer and de Smit, 1977, p34).
Systems Engineering: The formally managed design, build, and maintenance of man-made systems. [In fact, this term is usually only applied to information systems; when dealing with physical systems, the process tends simply to be called "engineering", or one of its 40-plus particular incarnations such as "civil engineering", "marine engineering", etc.]
Systems Theory: "The transdisciplinary study of the abstract organisation of phenomena, independent of their substance, type, or spatial or temporal scale of existence. It investigates both the principles common to all complex entities, and the (usually mathematical) models which can be used to describe them." (Heylighen and Joslyn, 1992/2003 online)
3 - References