11/1 DESIGN FOR A BRAIN 



in the living organism's defences has, in fact, long been known 

 and made use of by the hunter. The stalking cat's movements 

 are such as will maintain as long as possible, for the prey, 

 the appearance of a peaceful landscape, to be changed with 

 the utmost possible suddenness into one of mortal threat. In the 

 whole process the suddenness is essential. Consider too the 

 essential features of any successful trap ; and the necessity, in 

 poisoning vermin, of ensuring that the first dose is lethal. 



If, then, the ultrastable system usually fails when attempting to 

 adapt to an environment with sudden discontinuities, so too does 

 the living organism. 



(3) Another weakness shown by the ultrastable system's method 

 is that success is dependent on the system's using a suitable 

 period of delay between each trial. Thus, the system shown in 

 Fig. 8/7/1 must persist in Trial IV long enough for the repre- 

 sentative point to get away from the region of the critical states. 

 Both extremes of delay may be fatal : too hurried a change from 

 trial to trial may not allow time for i success ' to declare itself ; 

 and too prolonged a testing of a wrong trial may allow serious 

 damage to occur. Up to now I have said nothing of this necessity 

 for delay between one trial and the next, but there is no doubt 

 that it is an essential part of the ultrastable system's method 

 of adaptation. Thus the homeostat needed a device, not shown 

 in Fig. 8/8/3, for allowing the uniselectors to move only at about 

 every 2-3 seconds. 



In animals, little is known scientifically about the optima for 

 such delays. But there can be little doubt that on many occa- 

 sions living organisms have missed success either by abandoning 

 a trial too quickly, or by persisting too long with a trial that 

 was actually useless. 



The same difficulty, then, seems to confront both ultrastable 

 system and living organism. 



(4) If we grade an ultrastable system's environments according 

 to the difficulty they present, we shall find that at the ' easy ' 

 end are those that consist of a few variables, independent of each 

 other, and that at the ' difficult ' end are those that contain many 

 variables richly cross-linked to form a complex whole. 



The living organism, too, would classify environments in 

 essentially the same way. Not only does common experience 

 show this, but the construction and use of ' intelligence tests ' 



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