5/8 DESIGN FOR A BRAIN 



5/8. We can now recognise that 4 adaptive ' behaviour is equi- 

 valent to the behaviour of a stable system, the region of the 

 stability being the region of the phase-space in which all the 

 essential variables lie within their normal limits. 



The view is not new (though it can now be stated with more 

 precision) : 



1 Every phase of activity in a living being must be not 

 only a necessary sequence of some antecedent change in its 

 environment, but must be so adapted to this change as to 

 tend to its neutralisation, and so to the survival of the 

 organism. ... It must also apply to all the relations of 

 living beings. It must therefore be the guiding principle, 

 not only in physiology . . . but also in the other branches 

 of biology which treat of the relations of the living animal 

 to its environment and of the factors determining its survival 

 in the struggle for existence.' 



(Starling.) 



1 In an open system, such as our bodies represent, com- 

 pounded of unstable material and subjected continuously to 

 disturbing conditions, constancy is in itself evidence that 

 agencies are acting or ready to act, to maintain this con- 

 stancy.' 



(Cannon.) 



4 Every material system can exist as an entity only so long 

 as its internal forces, attraction, cohesion, etc., balance the 

 external forces acting upon it. This is true for an ordinary 

 stone just as much as for the most complex substances ; 

 and its truth should be recognised also for the animal organism. 

 Being a definite circumscribed material system, it can only 

 continue to exist so long as it is in continuous equilibrium 

 with the forces external to it : so soon as this equilibrium 

 is seriously disturbed the organism will cease to exist as the 

 entity it was.' 



(Pavlov.) 



McDougall never used the concept of 4 stability ' explicitly, but 

 when describing the type of behaviour which he considered to 

 be most characteristic of the living organism, he wrote : 



c Take a billiard ball from the pocket and place it upon the 

 table. It remains at rest, and would continue to remain so 

 for an indefinitely long time, if no forces were applied to it. 

 Push it in any direction, and its movement in that direction 

 persists until its momentum is exhausted, or until it is 

 deflected by the resistance of the cushion and follows a new 



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