4/15 DESIGN FOR A BRAIN 



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. 



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 resting state (which, in these examples, 

 can be moved by an outside operation). The thermostat is 

 affected by the difference between the actual and the set tem- 

 peratures. The searchlight is affected by the difference between the 

 two directions. So it will be seen that 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. 



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 ' or ' intelligent ' 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 mini- 

 mise 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 feed- 

 back may be both wholly automatic and yet actively and complexly 

 goal-seeking. There is no incompatibility. 



4/15. An important feature of a system's stability (or instability) 

 is that it is a property of the whole system and can be assigned 

 to no part of it. The statement may be illustrated by a con- 

 sideration of the third diagram of S. 4/12 as it is related to the 

 practical construction of the thermostat. In order to ensure the 

 stability of the final assembly, the designer must consider : 

 (1) The effect of the temperature on the diameter of the cap- 

 sule, i.e. whether a rise in temperature makes the capsule 

 expand or shrink. 



54 



