Basic Notions - Variety and the Law of Requisite Variety

To control—or manage[1]— anything, there is a need for something to measure it with; so as to control complexity, we need a measure to understand the size of a given complex situation. In Cybernetics, Ashby[2] introduces the concept of variety: “The number of possible distinct states of a system.” The more complex a system gets, the more possible states it can produce, and the greater its variety becomes (and vice versa).

Ashby’s Law of Requisite Variety states that (p. 207) “only variety can destroy variety.”[3] In other words, to control a process with a certain amount of variety, we need a control/management function with at least the same amount of variety, or “the larger the variety of actions available to a control system, the larger the variety of perturbations it is able to compensate.”[4]

As an example, to control the temperature of a room, we need a control function/device, say a thermostat, so as to create a comfortable environment. If the outside temperature is cold (e.g., in wintertime) and the room temperature has only two states for the observer, say ‘comfortable’ and ‘uncomfortable,’ the system in focus has a low variety; it’s either comfortable or not: A variety of 2. Therefore, according to Ashby’s Law of Requisite Variety, to control this system, we need a control device (i.e., a thermostat) with the minimum variety of 2: To turn ‘on’ and ‘off’ the flow of warm air to the room, keeping the temperature of the room in a comfortable range.

Increasing the variety, let’s assume a more complex system: What if the “comfort level” can change in time, and by doing so, the system can better manage the load of the heating system when we are not using the room? In this situation, the environment has more than 2 possible states (i.e., the variety is more than 2), and we need a device with an appropriate number of varieties to control accordingly. The thermostat needs to be able to tell in what temperature the room should be in specific time ranges: Sleep time, morning time, out of the room, before coming back from work, coming back from work, etc.

The complexity of this system can be further increased by differentiating between working days and weekends; hence more variety to manage and a more complex thermostat.

Human beings are significantly high variety systems, and the implication of Ashby’s Law of Requisite Variety in a mediation situation is that the mediator needs to be able to control the combined variety of two human beings, at the least, and considerably more, if other stakeholders (e.g., friends, family, parties’ lawyers) are added to the mix. One can easily imagine how the complexity of the situation can increase exponentially. We will explore this complexity further.


[1] The terms “control” and “manage” are used interchangeably.

[2] Ashby, W. R. (1956, October). An Introduction to Cybernetics (Vol. 37). London: Chapman & Hall.

[3] While “destroy” is the word Ashby used in his book, the Law of Requisite Variety is also known as: “only variety can absorb variety.”

[4] Heylighen, F., & Joslyn, C. (2001, Aug 31). The Law of Requisite Variety. Retrieved from Principia Cybernetica:

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