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Home » Systems Thinking » What is Ross Ashby’s Law of Requisite Variety? (#systemsthinking background material)

What is Ross Ashby’s Law of Requisite Variety? (#systemsthinking background material)

I think you will spend 100 seconds reading this post

This is a concept that I’m using since quite some time now and that I seemed to understand rather intuitively though, necessarily at a general level.

What it basically says is that for a controller to remove noise from a signal, it needs to have a minimum variety that depends on the signal it needs to remove noise from and the variety of the result that it deems Good. Which Ross Ashby summarized as “only variety can kill variety“, where the killing part was about killing the variety of noise.

My first detailed encounters with the law of requisite variety from Ross Ashby were in his book “Introduction to Cybernetics” made available from the Principia Cybernetica web site (thanks to them!), more precisely chapter 11. I must confess that it was rather difficult to read.

Then, the other day, I came upon these Ross Ashby’s archives of all his journals, including his hand-made index cards. This is mind-blowing! quoting the web site:

Ross’s Journal consists of well over a million words on 7,400 pages in 25 volumes, and over 1,600 index cards.

Can you believe it?!

And so there is this index card on requisite variety which links to all the following pages of his journal on it (click on the red page numbers on the web site).

I find the explanations being somewhat easy to follow and historically very attractive.

And then I also found this other paper from Ashby titled “Requisite variety and its implications for the control of
complex systems” . I’m half way through it, but it is quite easy to follow as well. Check it out!

The law is pretty harsh knowledge for lay people like us, but what’s important to me is to know that it has been embedded by Stafford Beer in his Viable System Model for organizations, and this is much more directly useful to me! I might talk about it in some other posts!

Have a nice summer, dear readers!


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