DESIGN FOR A BRAIN 7/26 



of oxygen or salt concentration perhaps, or local strains. If the 

 reacting part R is initially formed by such a process, then the 

 action of the second feedback is unaffected: it will bring the 

 organism to adaptation. 



In the same way, nothing was supposed about the successive 

 values at S (except that they must not be appreciably correlated 

 with the events within the field). Any uncorrelated source will 

 therefore serve for their supply ; so they too can be, in the denned 

 sense, random. 



The ultrastable system 



7/26. In the first edition the system described in this chapter 

 was called ' ultrastable ', and S. 8/6 will show why the adjective 

 is defensible. At that time the system was thought to be unique, 

 but further experience (outlined in /. to C, S. 12/8-20) has shown 

 that this form is only one of a large class of related forms, in 

 which it is conspicuous only because it shows certain features, of 

 outstanding biological interest, with unusual clarity. The word 

 may usefully be retained, in accordance with the strategy of 

 S. 2/17, because it represents a well-defined type, useful as a fixed 

 type around which discussion may move without ambiguity, and 

 to which a multitude of approximately similar forms, occurring 

 mostly in the biological world, may be related. 



For convenience, its definition will be stated formally. Two 

 systems of continuous variables (that we called ' environment ' 

 and ' reacting part ') interact, so that a primary feedback (through 

 complex sensory and motor channels) exists between them. 

 Another feedback, working intermittently and at a much slower 

 order of speed, goes from the environment to certain continuous 

 variables which in their turn affect some step-mechanisms, the 

 effect being that the step-mechanisms change value when and 

 only when these variables pass outside given limits. The step- 

 mechanisms affect the reacting part; by acting as parameters to 

 it they determine how it shall react to the environment. 



(From this basic type a multitude of variations can be made. 

 Their study is made much easier by a thorough grasp of the 

 properties of the basic form just defined.) 



7/27. The basic form has many more properties of interest than 

 have yet been indicated. Their description in words, however, is 



98 



