Ashby’s 1952 work ‘Design for a Brain’ considered in depth the idea of adaptive behaviour and its working in the brain, and:

to show that a system can be both mechanistic in nature and yet produce behaviour that is adaptive.

(Ashby second edition, 1960 p1)

He was attempting to create a mechanistic model of the brain and its ability to learn. In the book, he introduced the concept of ‘ultrastability’, a form of double feedback which is present in any system which has adaptive behaviour altered by interaction with its environment. This concept later formed the basis of Argyris and Schön’s work on double-loop learning.

Ashby discussed the concept of variety: the number of possible states that a system can have, which in a system with more than a small number of variables will be a very large number. He applied the concept of variety to the question of regulation – the control, governance or management of a larger system by a smaller system – considering the relationship between the variety in the regulator and in the system that is being regulated. His Law of Requisite Variety said that the regulator must contain as much variety as the system being regulated. As Ramage and Shipp put it (2009 p48):

If there is insufficient variety in the regulator, the only solutions are either to increase the variety in the regulator or to decrease the variety in the system being regulated it is not the case that the variety of the regulator must be identical to the variety of the system regulated, but there is a strict mathematical (logarithmic) relationship between the two, and the regulator must contain sufficient variety to cope.

Jackson (2003 p9) states it differently:

Systems can only be controlled if the would-be-controller can command the same degree of variety as the system. Today systems are complex and change rapidly; they exhibit high variety. Managers need to pay attention to reducing the variety of the system they are seeking to control and/or to increasing their own variety. This process of balancing varieties is known as variety engineering.

Ashby was an innovator, laying the foundations for many systems thinkers who followed including Stafford Beer, von Bertalanffy, Chris Argyris and Donald Schön.  His books on cybernetics were considered landmarks in the field and were highly influential.


Ashby, W.R. 1956 ‘An Introduction to Cybernetics’. Methuen, London.

Ashby W.R. 1958 ‘Requisite variety and its implications for the control of complex systems’, Cybernetica 1:2, p. 83-99

Ashby W.R. 1960 ‘Design for a Brain: the origin of adaptive behaviour’ (2nd edition) Chapman and Hall, London

Ramage M and Shipp K 2009 ‘Systems Thinkers’ Open University: Milton Keynes