3/13 DESIGN FOR A BRAIN 



in the body than the tongue : the child has to learn how to play 

 without exhausting itself utterly, and how to talk without getting 

 out of breath. 



These remarks are not intended to confuse, but to show that 

 later arguments (S. 17/4 and Chapter 18) are not unreasonable. 

 There it is intended to treat one group of neurons in the cerebral 

 cortex as the environment of another group. These divisions, 

 though arbitrary, are justifiable because we shall always treat the 

 system as a whole, dividing it into parts in this unusual way merely 

 for verbal convenience in description. 



It should be noticed that from now on w the system ' means 

 not the nervous system but the whole complex of the organism 

 and its environment. Thus, if it should be shown that i the 

 system ' has some property, it must not be assumed that this 

 property is attributed to the nervous system : it belongs to the 

 whole ; and detailed examination may be necessary to ascertain 

 the contributions of the separate parts. 



3/13. In some cases the dynamic nature of the interaction 

 between organism and environment can be made intuitively more 

 obvious by using the device, common in physics, of regarding the 

 animal as the centre of reference. In locomotion the animal 

 would then be thought of as pulling the world past itself. Pro- 

 vided we are concerned only with the relation between these two, 

 and are not considering their relations to any third and inde- 

 pendent body, the device will not lead to error. It was used 

 in the i rider and bicycle ' example. 



By the use of animal-centred co-ordinates we can see that the 

 animal has much more control over its environment than might at 

 first seem possible. Thus when a dog puts its foot on a sharp and 

 immovable stone, the latter does not seem particularly dynamic. 

 Yet the dog can cause great changes in this environment — by 

 moving its foot away. Again, while a frog cannot change air into 

 water, a frog on the bank of a stream can, with one small jump, 

 change its world from one ruled by the laws of mechanics to one 

 ruled by the laws of hydrodynamics. 



Static systems (like the sharp stone) can always be treated as if 

 dynamic (though not conversely), for we have only to use the 

 device of S. 2/3 and treat the static variable as one which is 

 undergoing change of zero degree. The dynamic view is therefore 



40 



