048 The Law of Requisite Variety (Ashby’s Law)

If the law of requisite variety is to be handled intelligently, and not just by leaving nature to find the variety balance (which of course can be nasty for us humans), then it follows that the regulative forces must not only dispose requisite variety — which is a number of possible states; they must also know the pattern by which variety in the system is deployed. On the journey to work we need to have enough options open; we also need to know the pattern of the highways- where they run, what the control points are like, what other drivers habitually do. In the process of putting the children to bed we need several variety amplifiers at our command; but we also need to know (as we do, but let’s make it explicit) the likely behaviour pattern of the children. Without these known patterns, proliferating variety looks even more threatening than it really is, which is bad enough.

What I have been calling a pattern is what a scientist calls a model. A model is not a load of mathematics, as some people think; nor is it some unrealizable ideal, as others believe. It is simply an account — expressed as you will — of the actual organization of a real system. Without a model of the system to be regulated, you cannot have a regulator. That’s the point. And you can test that too.

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The text of six radio broadcasts given in the autumn of 1973 as the thirteenth series of Massey Lectures which were established in 1961 by the Canadian Broadcasting Corporation to enable distinguished authorities in fields of general interest and importance to present the results of original study or research.