The art of thinking · Parts, wholes and systems
Law of requisite variety
Also known as: Ashby’s law, Requisite variety, Law of requisite variety (W. Ross Ashby)
German: Gesetz der erforderlichen Varietät
In cybernetics, the law of requisite variety, formulated by W. Ross Ashby in 1956, states that a regulator can only keep a system’s outcomes within limits if it can produce at least as many distinct responses as there are distinct disturbances to counter — “only variety can destroy variety”.
- Thinking models
- Systems theory
In one sentence
Ashby’s law: a regulator can only control what it can match in variety — only variety can destroy variety.
Example
A single generic troubleshooting procedure for twenty different fault codes can’t resolve them all; the documentation needs as many distinct responses as there are distinct situations that call for different action.
How it applies
- The law: Variety is the number of distinguishable states. If disturbances can take more forms than the regulator has responses, some disturbances get through. Reducing the variety of outcomes requires matching variety in the regulator — or reducing the variety of the disturbances.
- Two ways to comply: Increase the regulator’s repertoire (more responses, more skilled operators) or attenuate the incoming variety (standardize, filter, constrain the situation). Organizations usually need both.
- Later use: Stafford Beer built his management cybernetics on Ashby’s law; it is also cited in safety science and organization theory.
- Technical documentation: Instructions are part of the regulator of a work system. Where situations differ in ways that require different action, the documentation must distinguish them; where they don’t, merging them reduces needless variety for the reader.
Requisite variety vs. completeness
The law does not ask for documentation of everything; it asks that every difference that requires a different response is covered. Differences that change nothing for the reader are noise.
How to read this pool
Entries, context cards and transitional topics
The art of thinking explains ways of thinking — where a method comes from, what it asks of you and how it was adapted later. You meet four kinds of content here, each with its own job:
- Glossary entry
- Defines one concept or method of thinking: its origin, an example and how it relates to neighboring ideas. “X vs. Y” sections set two methods side by side.
- Context card of the art of thinking
- A short, self-contained answer to one question about a way of thinking — its lineage, its steps, its limits. These cards are listed on The art of thinking only.
- Transitional topicBridges both pools
- A context card that bridges both pools: it shows how a way of thinking was adapted into documentation, analysis or quality practice. It sits with the reference context cards and is marked as transitional wherever it appears.
- Reference entry or context card
- Belongs to the Glossary & Norms Space on regulation, norms and engineering. Entries here link to one only where a method is also an established tool of documentation or quality work, such as the five whys or a user’s mental model.