11/4 DESIGN FOR A BRAIN 



Let the homeostat be arranged so that it is partly under uni- 

 selector-, and partly under hand-, control. Let it be started so 

 that it works as an ultrastable system. Select a commutator 

 switch, and from time to time reverse its polarity. This reversal 

 provides the system with the equivalent of two environments 

 which alternate. We can now predict that it will be selective for 

 fields that give adaptation to both environments. For consider 

 what field can be terminal : a field that is terminal for only one of 

 the parameter- values will be lost when the parameter next changes; 

 but the first field terminal for both will be retained. Figure 

 11/4/1 illustrates the process. At R 19 R 2 , jR 3 , and R± the hand- 



Jj J, Rj * 3 R4 



V 



lA A. 



* H- 



X h 3l- 



Time 



Figure 11/4/1 : Record of homeostat's behaviour when a commutator H 

 was reversed from time to time (at the R's). The first set of uniselector 

 values which gave stability for both commutator positions was terminal. 



controlled commutator H was reversed. At first the change of 

 value caused a change of field, shown at A. But the second 

 uniselector position happened to provide a field which gave 

 stability with both values of H. So afterwards, the changes of 

 H no longer caused step-function changes. The responses to the 

 displacements Z), forced by the operator, show that the system 

 is stable for both values of //. The slight but distinct difference 

 in the behaviour after D at the two values of H show that the 

 two fields are different. 



The ultrastable system is, therefore, selective for step-function 

 values which give stability for both values of an alternating 

 parameter. 



11/5. Such a process would occur in a biological system if an 

 animal had to adapt by one internal arrangement to two environ- 



1.34 



