International Encyclopedia of Systems and Cybernetics

2nd Edition, as published by Charles François 2004 Presented by the Bertalanffy Center for the Study of Systems Science Vienna for public access.


The International Encyclopedia of Systems and Cybernetics was first edited and published by the system scientist Charles François in 1997. The online version that is provided here was based on the 2nd edition in 2004. It was uploaded and gifted to the center by ASC president Michael Lissack in 2019; the BCSSS purchased the rights for the re-publication of this volume in 200?. In 2018, the original editor expressed his wish to pass on the stewardship over the maintenance and further development of the encyclopedia to the Bertalanffy Center. In the future, the BCSSS seeks to further develop the encyclopedia by open collaboration within the systems sciences. Until the center has found and been able to implement an adequate technical solution for this, the static website is made accessible for the benefit of public scholarship and education.


VARIETY (Requisite) (Law of)

"Only variety can destroy variety" (W.R. ASHBY, 1956" p.207)… or

"R's Capacity as a regulator cannot exceed its capacity as a channel for variety" (W. R. ASHBY, 1951, p.171).

In other words, the efficiency of a regulator in resisting environmental disturbances depends of the number of countervailing options it can produce.

For a system, this means that its regulators are able to maintain the system organization only to the extend that they can produce alternative states for any perturbation imposed by environmental noise.

Explain H. ATLAN: "This law is important… to understand the minimal conditions of structure needed for the survival of any system depending on an environmental source of aggressions and random perturbations.

"Let us consider a system exposed to a number of different possible perturbations. It has at is disposal a number of responses. Among the multiple possible states, only some are "acceptable" in view of the finality (at least apparent) of the system, which may be its sheer survival or the accomplishment of some function. The regulation is meant as choosing among the possible responses, those that will place the system in an acceptable state. ASHBY's law states a relation between the variety of perturbations, the one of responses and the one of acceptable states. Variety of responses must be bigger if variety of perturbations is big and the one of acceptable states small. In other words, a great variety of available responses is needed in order to insure a regulation in a system such as to maintain it in a very limited number of states while submitted to a great variety of aggressions. Still more, in any environment being a source of variable and unforeseable aggressions, variety in the structures and functions of the system is an requisite factor of autonomy" (1972, p.25).

This celebrated law was proposed by W.R. ASHBY in 1956. R. VALLÉE comments it in the following way: "Schematically, this principle, which is still controversial, as restated by R. CONANT (1969, p.334-6), says that the ability of a feedback device (for example a regulator) to resist successfully to a perturbing noise, increases with its capacity to transmit a greater quantity of information, per time unit, through its feedback chain. We have here a concept derived from SHANNON's theory (the capacity to transmit information) and an idea coming from games theory (the ability to resist to an opponent), the variety requisite for achieving the chosen purpose, being the variety of informations transmitted per time unit" (1993a, p.88).

In very complex interrelations between a system and its environment, it is not possible for the system to obtain absolute control over its environment because there is an upper limit to the variety that the system may contain.


  • 1) General information
  • 2) Methodology or model
  • 3) Epistemology, ontology and semantics
  • 4) Human sciences
  • 5) Discipline oriented


Bertalanffy Center for the Study of Systems Science(2020).

To cite this page, please use the following information:

Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]

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