Requisite Variety

Ashby’s Law of Requisite Variety:

If a system is to be stable, the number of states of its control mechanism must be greater than or equal to the number of states in the system being controlled.


Only variety can absorb variety.

Variety is a measure of the number of distinct states a system can be in. For a system to control another system, it needs to have at least as many distinct states as the system being controlled. This is called requisite variety.


The amount of output from a system is limited by the variety possible within the system and/or the variety of input to the system. The number of possible alternative communications between the two systems is limited by that system having the fewest output alternatives and/or the fewest input alternatives.

Charles E. Osgood

This is more than a design pattern, it is a generalizable law.

Two ways to achieve variety

There are two ways to achieve requisite variety:

Variety must be embodied

Knowledge in a system is embodied in the structure of that system. Therefore, the complexity of an adaptive system must match the complexity of that system’s environment.

If you want to make sense of a complex world, you've got to have an internal system that is equally complex.

Karl Weick

Limiting variety is authoritarian

To be authoritarian is to choose methods of control that limit variety.

Any ideology that flattens the complexity of human life down to a simple system will embody less variety than the reality of human experience.

Static systems must limit variety to remain stable

The only way to maintain stability in a static system is by limiting variety. This makes static systems authoritarian.

Even then, the environment will continue changing.

By contrast, living systems are dynamic. They coevolve with the environment, and with the agents that make them up.

Instead of limiting the variety of people, increase the variety of the system.