ASHBY’S LAW OF REQUISITE VARIETY

William Ross Ashby (1903-1972) was a British pioneer in the fields of cybernetics and systems theory. He studied at the Edinburgh Academy and Cambridge University, and most of his life's employment was in medical institutions as a psychiatrist. In 1960, he went to the United States and became professor in the fields of biophysics and electrical engineering; but he was also president of the Society for General Systems Research from 1962 to 1964. He became a Honorary Professorial Fellow at the University of Wales in 1970 and a fellow of the Royal College of Psychiatrists in 1971. In June 1972 he was diagnosed with an inoperable brain tumor and died that year.

William Ross Ashby

He is best known for the law of requisite variety, which states:

Variety absorbs variety, defines the minimum number of states necessary for a controller to control a system of a given number of states.

This rather technical statement could be modified to make it more understandable as:

The greater the variety of actions available to us when we seek exert control, the greater the variety of disturbances we are able to control.

- so, as in our car journey example: the great variety of kit, gadgets etc. that I take with me, the greater variety of disturbing situations I am able to control.

And this is exactly where the limitations of us human beings comes in when we seek to exert control. As we saw above, the human brain is a machine with a limited size and it therefore has limits to its ability to deal with complexity. As Stafford Beer, a follower of Ashby's thinking, wrote:

The first thing we have to face up to is quite a tough proposition for people reared in our culture. It is that whatever we humans can do is mediated by our brains, and those brains are finite. We have in the cranium a slightly alkaline three-pound electrochemical computer running on glucose at about 25 watts. This computer contains some ten thousand million logical elements called neurons, operating on a basic scanning rhythm of ten cycles per second. This is a high-variety dynamic system all right; but it really is finite. It follows from Ashby's Law that we can recognize patterns up to a certain limit, and not beyond. Thus if something is going on that involves a higher variety than the brain commands, we shall not recognize what it is. This is the old constraint of requisite variety again.

Stafford Beer

There are practical consequences to this. For instance, I am sure that the reason why we are making such a hash of the problems of global ecology is that we cannot understand them. I don't just mean that they are awfully difficult, so that understanding will take a lot of research. I mean that we cannot understand at all, ever . . .

This then brings us to an essential point about 'MANAGEMENT'—>