p.1 Given a set of elements, its variety is the number of elements that can be distinguished. Thus the set {g b c g g c } has a variety of 3 letters.

p.1 Regulation achieves a "goal" against a set of disturbances.

p.2 the disturbances D might be all the attacks that can be made by a hostile army, and the responses R all the counter-measures that might be taken... The "outcomes" so far are simple events, without any implication of desirability. In any real regulation, for the benefit of some defined person or organism or organisation, the facts usually determine a further mapping of the set Z of outcomes into a set E of values... In the living organisms, the scale of values is usually related to their "essential variables" - those fundamental variables that must be kept within certain "physiological" limits if the organism is to survive.

p.3 a suitable regulator that has access to the disturbance may be able to counter its effects, remove its effect from y, and thus leave y wholly under the control of a. In this case, successful regulation by R is the necessary and sufficient condition for successful control by a.

p.4 restriction of the outcomes to the subset that is valued as Good demands a certain variety in R... The law of requisite variety says that R's capacity as a regulator cannot exceed its capacity as a channel for variety.

p.5 In the biological world, examples that approximate to this form are innumerable, though few correspond with mathematical precision. This inexactness of correspondence does not matter in our present context, for we shall not be concerned with questions involving high accuracy, but only with the existence of this particular limitation.

An approximate example occurs when a organism is subject to attacks by bacteria (of species di) so that, if the organism is to survive, it must produce the appropriate antitoxin rj. If the bacterial species are all different, and if each species demands a different anti-toxin, then clearly the organism, for survival, must have at least as many anti-toxins in its repertoire of responses as there are bacterial species.

Again, if a fencer faces an opponent who has various modes of attack available, the fencer must be provided with at least an equal number of modes of defence if the outcome is to have the single value: attack parried.

p.5 [Sommerhoff (1950)] recognises that the concept of "regulation" demands variety in the disturbances D... His work shows, irrefutably in my opinion, how the concepts of co-ordination, integration, and regulation are properly represented in abstract form by a relation between the coenetic variable [whatever is responsible for the values of disturbances D] and the response, such that the outcome of the two is the achievement of some "focal condition"... From our point of view, what is important is the recognition that without the regulatory response the values at the focal condition would be more widely scattered. [JLJ - Sommerhoff seems to have beaten everyone to the idea of critical success factors or essential variables. Unfortunately, "coenetic variable" is not a name that survived in association with the idea.]

p.6 Let me summarise what has been said about "regulation". The concept of regulation is applicable when there is a set D of disturbances, to which the organism has a set R of responses, of which on any occasion it produces some one, rj say. The physico-chemical or other nature of the whole system then determines the outcome. This will have some value for the organism, either Good or Bad say. If the organism is well adapted, or has the know-how, its response rj, as a variable, will be such a function of the disturbance di that the outcome will always lie in the subset marked as Good.

p.9 the design engineer may sometimes forget that there is another way to regulation. May I suggest that he would do well to bear in mind what has been found so advantageous in the biological world, and to consider whether a regulation which is excessively difficult to design when it is controlled by error may not be easier to design if it is controlled not by the error but by what gives rise to the error.

p.9 without an agreed valuation the concept of regulation does not exist.

p.10 Consider, for instance, the repeated attempts that used to be made (especially in the last century) in which some large Chess Club played the World Champion. Usually the Club had no better way of using its combined intellectual resources than either to take a simple majority vote on what move to make next (which gave a game both planless and mediocre), or to follow the recommendation of the Club's best player (which left all members but one practically useless). Both these methods are grossly inefficient.

p.12 Operational research... Its first characteristic is that its ultimate aim is not understanding but the purely practical one of control. If a system is too complex to be understood, it may nevertheless still be controllable. For to achieve this, all that the controller wants to find is some action that gives an acceptable result; he is concerned only with what happens, not with why it happens.

p.12 if a variation works, exploit it further; ask not why it works, only if it works.

p.13 I suggest that recognition of the limitation implied by the law of requisite variety may, in time, also prove useful, by ensuring that our scientific strategies for the complex system shall be, not slavish and inappropriate copies of the strategies used in physics and chemistry, but new strategies, genuinely adapted to the special peculiarities of the complex system.