## The idea in brief The **Law of Requisite Variety**, formulated by cybernetician **[[W. Ross Ashby]]** in the 1950s, is a principle of control and regulation in complex systems. It states that _“only variety can absorb variety”_. In other words, a regulator must be at least as flexible and diverse in its responses as the challenges it faces from its environment. This idea became a cornerstone of **cybernetics** and later shaped organisational theories such as Stafford Beer’s **[[Viable System Model]] (VSM)**. --- ## Key concepts ### Variety - **Variety** refers to the number of possible states a system can take. - For example, if an environment presents ten different challenges, a regulator must be capable of responding in at least ten different ways. ### The law - **Core statement**: A system can only remain stable if its control mechanisms have enough variety to match the disturbances it encounters. - If not, external disturbances will overwhelm it, leading to instability or failure. ### Regulation - Effective regulation reduces the complexity of an environment by filtering or adapting. - Regulation can be achieved by: 1. **Attenuation** – reducing external variety (e.g. setting rules to limit unpredictable behaviour). 2. **Amplification** – increasing the regulator’s variety (e.g. training staff to handle new situations). ### Link to cybernetics - [[Cybernetics]] is concerned with control, communication, and feedback in systems. - Ashby's law formalises the conditions under which control is possible. --- ## Sources - [[The Unaccountability Machine]] — Dan Davies applies requisite variety to explain why organisations lose the ability to adapt - [[Systemantics]] — Gall's complementary insight: systems grow to fill available variety, then become rigid - [[Requisite Organization]] — Elliott Jaques on matching organisational complexity to task complexity