DESIGN FOR A BRAIN 



12/12 



anaesthetised, or frozen; what is necessary is that the segment 

 should be unvarying. 



Physical separation, already noticed to give no certain inde- 

 pendence, is sometimes effective because it sometimes creates an 

 intervening region of constancy. 



12/12. The example of Figure 12/10/1 showed one way in which 

 the behaviour of a set of variables, by sometimes fluctuating and 

 sometimes being constant, could affect the independencies within 

 a system. The range of ways is, however, much greater. 



To demonstrate the variety we need a rule by which we can 

 make the appropriate modifications in the diagram of ultimate 

 effects when one or more of the variables is held constant. The 

 rule is: — Take the diagram of immediate effects. If a variable V 

 is constant, remove all arrows whose heads are at V; then, 

 treating this modified diagram as one of immediate effects, com- 

 plete the diagram of ultimate effects, using the rule of S. 12/9. 

 The resulting diagram will be that of the ultimate effects when 



lixiixu/i 



« 4-f==r3 4 + 3 4 



Figure 12/12/1 : If a four- variable system has the diagram of immediate 

 effects A, and if 1 and 2 are part-functions, then its diagram of ultimate 

 effects will be B, C, D or E as none, 1, 2, or both 1 and 2 become inactive, 

 respectively. 



V is constant. (It will be noticed that the effect of making V 

 constant cannot be deduced from the original diagram of ultimate 

 effects alone.) Thus, if the system of Figure 12/12/1 has the 

 diagram of immediate effects A, then the diagram of ultimate 

 effects will be B, C, D or E according as none, 1, 2, or both 1 and 

 2 are constant, respectively. 



It can be seen that with only four variables, and with only 

 two of the four possibly becoming constant, the patterns of inde- 



166 



