Ashby's law, also known as the Law of Requisite Variety, is a principle in cybernetics and systems theory proposed by British scientist W. Ross Ashby in the mid-20th century. The law states that the variety (complexity) of a control system must match or exceed the variety of the system being controlled in order to effectively regulate it.
In other words, if a system is complex and has many possible states, then the control system regulating it must also be complex enough to accommodate all those possible states. If the control system is not complex enough, it will be unable to regulate the system effectively, and the system may become unstable or unpredictable.
This principle is important in fields such as engineering, management, and psychology, where systems are designed and controlled in order to achieve specific outcomes. By ensuring that the control system has requisite variety, designers and managers can help ensure that the system operates in a stable and predictable manner.