STABILITY 



4/14 



of change is deceptive if it suggests rigidity : they have only to be 

 disturbed to show that they are capable of extensive and active 

 movements. They are restricted only in that they do not show 

 the unlimited divergencies of instability. 



Goal-seeking 



4/14. Every stable system has the property that if displaced 

 from a resting state and released, the subsequent movement is 

 so matched to the initial displacement that the system is brought 

 back to the resting state. 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 



— Time 



Figure 4/14/1 : Tracing of the temperature (solid line), of a thermostatically 

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



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 4/14/1 shows how, as the control setting of a thermostat 

 was altered, the temperature of the apparatus always followed it, 

 the set temperature being treated as if it were a goal. 



Such a movement occurs here in only one dimension (tempera- 



53 E 



