13/4 DESIGN FOR A BRAIN 



a displacement that destroyed A's. Similarly a displacement 

 applied to the representative point on the resting cycle in B is 

 more likely to change the field than if applied to C. A field like 

 C, therefore, with its resting states compact and near the centre 

 of the region, tends to have a higher immunity to displacement 

 than fields whose resting states or cycles go near the edge of the 



Figure 13/4/1 : Three fields of an ultrastable system, differing in their 

 liability to change when the system is subjected to small random dis- 

 turbances. (The critical states are shown by the dots.) 



region. (A quantitative discussion of the tendency is given in 

 S. 23/4.) 



If the disturbances fall on a large number of iterated ultrastable 

 systems, the probabilities become actual frequencies. We can 

 then predict that if iterated ultrastable systems are subjected to 

 repeated small occasional and random disturbances, the average 

 terminal field will tend to the form C. 



13/5. How would this tendency show itself in the behaviour of 

 the living organism ? 



In S. 8/7 we noticed that a field may be terminal and yet 

 show all sorts of bizarre features : cycles, resting states near the 

 edge of the region, stable and unstable lines mixed, multiple 

 resting states, multiple resting cycles, and so on. These possi- 

 bilities obscured the relation between a field's being terminal and 

 its being suitable for keeping essential variables within normal 

 limits. But a detailed study was not necessary ; for we have 

 just seen that all such bizarre fields tend selectively to be destroyed 

 when the system is subjected to small, occasional, and random dis- 

 turbances. Since such disturbances are inseparable from practical 

 existence, the process of ' roughing it ' tends to cause their replace- 



146 



