DESIGN FOR A BRAIN 



4/16 



4/15/1 (A). It is easy to verify that such a system contains 

 twenty interlaced circuits, two of which are shown at B and C. 



The further development of the theory of systems with feed- 

 back cannot be made without mathematics. But here it is 

 sufficient to note two facts: a system which possesses feedback 

 is usually actively stable or actively unstable; and whether it is 

 stable or unstable depends on the quantitative details of the 

 particular arrangement. 



Goal-seeking 



4/16. Every stable system has the property that if displaced 

 from a state of equilibrium and released, the subsequent movement 

 is so matched to the initial displacement that the system is 

 brought back to the state of equilibrium. A variety of disturbances 

 will therefore evoke a variety of matched reactions. Reference to a 

 simple field such as that of Figure 4/5/1 will establish the 

 point. 



This pairing of the line of return to the initial displacement 

 has sometimes been regarded as ' intelligent ' and peculiar to living 

 things. But a simple refutation is given by the ordinary pen- 

 dulum: if we displace it to the right, it develops a force which 

 tends to move it to the left; and if we displace it to the left, it 

 develops a force which tends to move it to the right. Noticing 

 that the pendulum reacted with forces which though varied in 

 direction always pointed towards the centre, the mediaeval scien- 

 tist would have said ' the pendulum seeks the centre '. By this 

 phrase he would have recognised that the behaviour of a stable 

 system may be described as ' goal-seeking '. Without introducing 

 any metaphysical implications we may recognise that this type of 

 behaviour does occur in the stable dynamic systems. Thus 

 Figure 4/16/1 shows how, as the control setting of a thermostat 



Figure 4/16/1 : Tracing of the temperature (solid line), of a thermostatically 

 controlled bath, and of the control setting (broken line). 



54 



