4/17 STABILITY 



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- 

 ture), but other goal-seeking devices may use more. The radar- 

 controlled searchlight, for example, uses the reflected impulses 

 to alter its direction of aim so as to minimise the angle between 

 its direction of aim and the bearing of the source of the reflected 

 impulses. So if the aircraft swerves, the searchlight will follow 

 it actively, just as the temperature followed the setting. Such 

 a system is goal-seeking in two dimensions. 



The examples show the common feature that each is ' error- 

 controlled ' : each is partly controlled by the deviation of the 

 system's state from the state of equilibrium (which, in these 

 examples, can be moved by an outside operation). The thermo- 

 stat is affected by the difference between the actual and the set 

 temperatures. The searchlight is affected by the difference 

 between the two directions. Thus, machines with feedback are not 

 subject to the oft-repeated dictum that machines must act blindly and 

 cannot correct their errors. Such a statement is true of machines 

 without feedback, but not of machines in general (S. 3/11). 



Once it is appreciated that feedback can be used to correct any 

 deviation we like, it is easy to understand that there is no limit 

 to the complexity of goal-seeking behaviour which may occur in 

 machines quite devoid of any ' vital ' factor. Thus, an automatic 

 anti-aircraft gun may be controlled by the radar-pulses reflected 

 back both from the target aeroplane and from its own bursting 

 shells, in such a way that it tends to minimise the distance between 

 shell-burst and plane. Such a system, wholly automatic, cannot 

 be distinguished by its behaviour from a humanly operated gun: 

 both will fire at the target, following it through all manoeuvres, 

 continually using the errors to improve the next shot. It will be 

 seen, therefore, that a system with feedback may be both wholly 

 automatic and yet actively and complexly goal-seeking. There 

 is no incompatibility. 



4/17. It will have been noticed that stability, as defined, in no 

 way implies fixity or rigidity. It is true that the stable system 

 usually has a state of equilibrium at which it shows no change; 

 but the lack of change is deceptive if it suggests rigidity: if dis- 

 placed from the state of equilibrium it will show active, perhaps 



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