William Ross Ashby

Cybernetics and Requisite Variety
An Introduction to Cybernetics




These are passages from Ross Ashby masterful book Cybernetics (1956).
The main purpose of this selection is to stimulate the reading of the full text (or of other relevant books on cybernetics) and to familiarize everybody with the Law of Requisite Variety.
This is a central law for the proper functioning of every mechanical and biological entity. It has been totally ignored by the social scientists and their patrons, the state elite, because it constitutes a refutation of the pretended absolute necessity of concentrating power in a central apparatus (the state) as the only way to solve problems (or, in general, to deal with reality) in a complex society.
In fact, the law expresses the exactly opposite view, declaring, with the support of logical reasoning and empirical evidence, that only variety can master variety, reducing disturbances and promoting harmonious order.
Regulation is then possible only if the regulating system is as various and flexible (responsive to changes) as the system to be regulated.
This principle then disposes of the myth (still cherished by journalists and sociologists in search of easy popularity) that extraordinarily complex situations demand the concentration of extraordinary powers in a central entity.
Once we get rid of that myth we are ready to explore all the rich implications of the Law of Requisite Variety and we, as individuals, can advance greatly towards finding real and appropriate solutions for the (supposedly) intractable problems of contemporary life.





1/1. Cybernetics was defined by Wiener as "the science of control and communication, in the animal and the machine" - in a word, as the art of steermanship. Co-ordination, regulation and control will be its themes, for these are of the greatest biological and practical interests.

1/2. Cybernetics deals with all forms of behaviour in so far as they are regular, or determinate, or reproducible.

1/3. Cybernetics stands to the real machine - electronic, mechanical, neural, or economic - much as geometry stands to a real object in our terrestrial space.

1/6. There are two peculiar scientific virtues of cybernetics, that are worth explicit mention.

One is that it offers a single vocabulary and a single set of concepts suitable for representing the most diverse types of systems. Until recently, any attempt to relate the many facts known about, say, servo-mechanisms to what was known about the cerebellum was made unnecessarily difficult by the fact that the properties of servo-mechanisms were described in words redolent of the automatic pilot, or the radio set, or the hydraulic brake, while those of the cerebellum were described in words redolent of the dissecting room and the bedside - aspects that are irrelevant to the similarities between a servo-mechanism and a cerebellar reflex. Cybernetics offers one set of concepts that, by having exact correspondences with each branch of science, can thereby bring them into exact relation with one other.

It has been found repeatedly in science that the discovery that two branches are related leads to each branch helping in the development of the other. The result is often a markedly accelerated growth of both. The infinitesimal calculus and astronomy, the virus and the protein molecule, the chromosomes and heredity are examples that come to mind. Neither, of course, can give proofs about the laws of the other, but each can give suggestions that may be of the greatest assistance and fruitfulness. Here I need only mention the fact that cybernetics is likely to reveal a great number of interesting and suggestive parallelisms between machine and brain and society. And it can provide the common language by which discoveries in one branch can readily be made use of in the others.

1/7. The complex system. The second peculiar virtue of cybernetics is that it offers a method for the scientific treatment of the system in which complexity is outstanding and too important to be ignored. Such systems are, as we well know, only too common in the biological world!

In the simpler systems, the methods of cybernetics sometimes show no obvious advantage over those that have long been known. It is chiefly when the systems become complex that the new methods reveal their power.

Science stands today on something of a divide. For two centuries it has been exploring systems that are either intrinsically simple or that are capable of being analysed into simple components. The fact that such a dogma as "vary the factors one at a time" could be accepted for a century, shows that scientists were largely concerned in investigating such systems as allowed this method; for this method is often fundamentally impossible in the complex systems. Not until Sir Ronald Fisher's work in the '20s, with experiments conducted on agricultural soils, did it become clearly recognised that there are complex systems that just do not allow the varying of only one factor at a time - they are so dynamic and interconnected that the alteration of one factor immediately acts as cause to evoke alterations in others, perhaps in a great many others. Until recently, science tended to evade the study of such systems, focusing its attention on those that were simple and, especially, reducible. In the study of some systems, however, the complexity could not be wholly evaded. The cerebral cortex of the free-living organism, the ant-hill as a functioning society, and the human economic system were outstanding both in their practical importance and in their intractability by the older methods. So today we see psychoses untreated, societies declining, and economic systems faltering, the scientist being able to do little more than to appreciate the full complexity of the subject he is studying. But science today is also taking the first steps towards studying "complexity" as a subject in its own right.

Prominent among the methods for dealing with complexity is cybernetics. It rejects the vaguely intuitive ideas that we pick up from handling such simple machines as the alarm clock and the bicycle, and sets to work to build up a rigorous discipline of the subject. For a time it seems rather to deal with truisms and platitudes, but this is merely because the foundations are built to be broad and strong. They are built so that cybernetics can be developed vigorously, without the primary vagueness that has infected most past attempts to grapple with, in particular, the complexities of the brain in action. Cybernetics offers the hope of providing effective methods for the study, and control, of systems that are intrinsically extremely complex. It will do this by first making out what is achievable , and then providing generalised strategies, of demonstrable value, that can be used uniformly in a variety of special cases. In this way it offers the hope of providing the essential methods by which to attack the ills - psychological, social, economic - which at present are defeating us by their intrinsic complexity.



The Law of Requisite Variety


8/1. … "variety", a concept inseparable from that of "information".

9/19. It must be noticed that noise is in no intrinsic way distinguishable from any other form of variety. Only when some recipient is given, who will state which of the two is important to him, is a distinction between message and noise possible.

10/1. The quantity of regulation that can be achieved is bounded by the quantity of information that can be transmitted in a certain channel.

11/2. The subject of regulation is very wide in its applications, covering as it does most of the activities in physiology, sociology, ecology, economics, and much of the activities in almost every branch of science and life. Further, the types of regulator that exist are almost bewildering in their variety. However, we shall be attempting to get at the core of the subject - to find what is common to all.

11/11. The Law of Requisite Variety ["Only variety can destroy variety."] enables us to apply a measure to regulation.

Let us reconsider what is meant, essentially, by 'regulation'.

There is first a set of disturbances D, that start in the world outside the organism, often far from it, and that threatens, if the regulator R does nothing, to drive the essential variables E outside their proper range of values.

Of all these E-values only a few (η) are compatible with the organism's life, or are unobjectionable, so that the regulator R, to be successful, must take its value in a way so related to that of D that the outcome is, if possible, always within the acceptable set η, i.e. within physiological limits.

We can now show these relations by the diagram of immediate effects:

T = exTernal world

D = Disturbances

E = Essential variables

R = Regulator




The arrows represent actual channels of communication. For the variety in D determines the variety in R; and that in T is determined by that in both D and R. If R and T are in fact actual machines, then R has an input from D, and T has two inputs.

We can now interpret the general phenomenon of regulation in terms of communication. If' R (Regulator) does nothing, i.e. keeps to one value, then the variety in D (Disturbances) threatens to go through T (exTernal world) to E (Essential variables), contrary to what is wanted. It may happen that T, without change by R, will block some of the variety and occasionally this blocking may give sufficient constancy at E for survival. More commonly, a further suppression at E is necessary; it can be achieved only by further variety at R.

We can now select a portion of the diagram, and focus attention on R as a transmitter:




The law of Requisite Variety says that R's capacity as a regulator cannot exceed R's capacity as a channel of communication.

In the form just given, the law of Requisite Variety can be shown in exact relation to Shannon's Theorem 10, which says that if noise appears in a message, the amount of noise that can be removed by a correction channel is limited to the amount of information that can be carried by that channel.

Thus, his "noise" corresponds to our "disturbances", his "correction channel" to our "regulator R", and his "message of entropy H" becomes, in our case, a message of entropy zero, for it is constancy that is to be "transmitted". Thus the use of a regulator to achieve homeostasis and the use of a correction channel to suppress noise are homologous.

11/13. The law now enables us to see the relations existing between the various types of variety and information that affect the living organism.

A species continues to exist primarily because its members can block the flow of variety (thought of as disturbance) to the gene-pattern, and this blockage is the species' most fundamental need. Natural selection has shown the advantage to be gained by taking a large amount of variety (as information) partly into the system (so that it does not reach the gene-pattern) and then using this information so that the flow via R blocks the flow through the environment T.

This point of view enables us to resolve what might at first seem a paradox - that the higher organisms have sensitive skins, responsive nervous systems, and often an instinct that impels them, in play or curiosity, to bring more variety to the system than is immediately necessary. Would not their chance of survival be improved by an avoidance of this variety?

The discussion in this chapter has shown that variety (whether information or disturbance) comes to the organism in two forms. There is that which threatens the survival of the gene-pattern - the direct transmission by T from D to E. This part must be blocked at all costs. And there is that which, while it may threaten the gene-pattern, can be transformed (or re-coded) through the regulator R and used to block the effect of the remainder (in T). This information is useful, and should (if the regulator can be provided) be made as large as possible; for, by the law of Requisite Variety, the amount of disturbance that reaches the gene-pattern can be diminished only by the amount of information so transmitted. That is the importance of the law in biology.

In its elementary forms the law is intuitively obvious and hardly deserving statement. If, for instance, a press photographer would deal with twenty subjects that are (for exposure and distance) distinct, then his camera must obviously be capable of at least twenty distinct settings if all the negatives are to be brought to a uniform density and sharpness.


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