REQUISITE VARIETY 11/15 



11/15. In our treatment of regulation the emphasis has fallen on 

 its property of reducing the variety in the outcome; without regula- 

 tion the variety is large — with regulation it is small. The limit of 

 this reduction is the regulation that holds the outcome rigorously 

 constant. This point of view is undoubtedly valid, but at first it 

 may seem to contrast sharply with the naive view that living organ- 

 isms are, in general, anything but immobile. A few words, in 

 addition to what was said in S. 11/1 3, may be useful. 



It should be appreciated that the distinction between "constant" 

 and "varying" often depends on the exact definition of what is 

 being referred to. Thus if a searchlight follows an aircraft accurately 

 we may notice either that the searchlight moved through a great 

 range of angles (angles in relation to the earth) or that the angle 

 it made with the aircraft remained constant at zero. Obviously 

 both points of view are valid; there is no real contradiction in this 

 example between "great range" and "constant", for they refer to 

 different variables. 



Again, the driver who steers a car accurately from one town to 

 another along a winding lane can be regarded either as one who has 

 caused the steering wheel to show much activity and change or as 

 one who, throughout the trip, has kept the distance between car and 

 verge almost constant. 



Many of the activities of living organisms permit this double aspect. 

 On the one hand the observer can notice the great deal of actual 

 movement and change that occurs, and on the other hand he can 

 observe that throughout these activities, so far as they are co- 

 ordinated or homeostatic, there are invariants and constancies that 

 show the degree of regulation that is being achieved. 



Many variations are possible on the same theme. Thus if variable 

 X is always doing just the same as variable y, then the quantity 

 X — y is constant at zero. So if j's values are given by some outside 

 factor, any regulator that acts on x so as to keep x — y constant 

 at zero is in fact forcing x to vary, copying y. Similarly, "making 

 X do the opposite to y" corresponds to "keeping x + j^ at some 

 constant value". And "make the variable u^ change so that it is 

 always just twice as large as v's (fluctuating) rate of change" corres- 

 ponds to "keep the quantity h' — 2dv/dt constant". 



It is a great convenience in exposition and in the processes of 

 general theory to be able to treat all "targets" as if they were of the 

 form "keep the outcome constant at a". The reader must, however, 

 not be misled into thinking that the theory treats only of immobility; 

 he must accustom himself to interchanging the corresponding con- 

 cepts freely. 



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