Ashby’s Law

The First Law of Cybernetics


W. Ross Ashby’s Law of Requisite Variety is so powerful that it is known as the First Law of Cybernetics.

Ashby’s Law implies that the degree of control of a system is proportional to the amount of information available. This means you need an appropriate amount of information to control any system, whatever it is.

This is a simple idea, though it is difficult to explain.


Variety is another way of thinking about information. It describes the number of potential states of a system – any system. If you recognise all the possible states, you have complete knowledge of the behaviour. Uncertainty occurs when you do not know all the possible states. As Ashby put it, variety is a concept inseparable from that of ‘information’.

Requisite Variety?

Requisite means necessary or required. So requisite variety implies that you need a certain amount of information for some purpose.

If you have complete knowledge of a system, it is possible to control it. However if the system has some hidden properties your information is incomplete and there is uncertainty about the behaviour. To have full control you need to have full knowledge of the system and its behaviour.

Ashby described it as

“Only variety destroys variety”

There are many similarly obscure descriptions such as

  • “The total number of possible states of a system, or of an element of a system.”
  • “The larger the variety of actions available to a control system, the larger the variety of perturbations it is able to compensate.”
  • ‘The greater the variety within a system, the greater its ability to reduce variety in its environment through regulation.”
  • “The quantity of regulation that can be achieved is bounded by the quantity of information that can be transmitted in a certain channel.”
  • “Variety absorbs variety”

We measure variety in bits. Alternately, we can measure it as a logarithmic measure (like information content). In this form variety represents the minimum number of choices (by binary chop) needed to resolve uncertainty.

When people first hear about Ashby’s law they often don’t get it – surely it’s trivial, obvious, everyone knows that.   Perhaps we all need thinking caps:

Ashby W.R. (1956) Introduction to Cybernetics, Chapman & Hall, Available free online.


6 Responses to Ashby’s Law

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  2. Andrew says:

    Interesting… don’t know anything about cybernetics. But I would have thought that even with full knowledge of a complex system you cannot predict its behaviour?? Equations become non-linear and can only be solved in special cases. So just because you have full knowledge of a system does not mean you can control it?

    • Great point. Basically, if the system is chaotic you cannot predict over time what it will do without complete knowledge of the system. In reality this can mean an infinite amount of information is needed. Weather forecasting is the classic example. You can get reasonably accurate predictions for a day or two but the errors increase with longer periods, and the prediction rapidly becomes worthless.

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