4/12 DESIGN FOR A BRAIN 



He would thus have established that there is an arrow from left 

 to right but not from right to left in 



Speed of 

 engine 



Distance 

 between 

 weights 



This procedure could then be applied to the two variables 

 ' distance between weights ' and ' velocity of flow of steam ', 

 while the other variable 4 speed of engine ' was kept constant. 

 And finally the relations between the third pair could be established. 



The method is clearly general. To find the immediate effects 

 in a system with variables A, B, C, D . . . take one pair, A and 

 B say ; hold all other variables C, D . . . constant ; note B's 

 behaviour when A starts, or is held, at A x ; and also its behaviour 

 when A starts, or is held, at A 2 . If these behaviours of B are the 

 same, then there is no immediate effect from A to B. But if the 

 B's behaviours are unequal, and regularly depend on what value 

 A starts from, or is held at, then there is an immediate effect, 

 which we symbolise by A — > B. 



By interchanging A and B in the process we can test for 

 B — > A . And by using other pairs in turn we can determine 

 all the immediate effects. The process is clearly defined, and 

 consists purely of primary operations. It therefore uses no 

 borrowed knowledge. We shall frequently use this diagram of 

 immediate effects. 



If A has an immediate effect on B, and B has an immediate 

 effect on A, the relation will be represented by A ^± B. If A 

 affects B, and B also affects C, but A does not affect C directly, the 

 relation will be shown by A — > B — > C. If there is a sequence of 

 arrows joined head to tail and we are not interested in the inter- 

 mediate steps, the sequence may often be contracted without 

 ambiguity to A — > C. The diagram will be used only for illustration 

 and not for rigorous proofs, so further precision is not required. (It 

 should be carefully distinguished from the diagram of ' ultimate ' 

 effects, but this is not required yet and will be described in S. 14/6. 

 At the moment we regard the concept of one variable ' having an 

 effect ' on another as well understood. But the concept will 

 be examined more closely, and given more precision, in S. 14/3.) 



A gas thermostat also shows a functional circuit or feedback ; 



50 



