THE MACHINE WITH INPUT 4/7 



SO that they become "coupled" as two automobiles may be locked 

 together after an accident. This form, however, is of little interest 

 to us, for the automobiles are too much changed by the process. 

 What we want is a way of coupling that does no violence to each 

 machine's inner working, so that after the coupling each machine 

 is still the same machine that it was before. 



For this to be so, the coupHng must be arranged so that, in prin- 

 ciple, each machine affects the other only by affecting its conditions, 

 i.e. by affecting its input. Thus, if the machines are to retain their 

 individual natures after being coupled to form a whole, the coupling 

 must be between the (given) inputs and outputs, other parts being 

 left alone no matter how readily accessible they may be. 



4/7. Now trace the operation in detail. Suppose a machine 

 (transducer) P is to be joined to another, R. For simplicity, 

 assume that P is going to affect R, without R affecting P, as when a 

 microphone is joined to an amplifier, or a motor nerve grows down 

 to supply an embryonic muscle. We must couple P's output to /?'s 

 input. Evidently i^'s behaviour, or more precisely the transforma- 

 tion that describes Rs, changes of state, will depend on, and change 

 with, the state of P. It follows that R must have parameters, for 

 input, and the values of these parameters must be at each moment 

 some function of the state of P. Suppose for definiteness that the 

 machine or transducer R has the three transformations shown in 

 S.4/1, i.e. 



and that P has the transformation, on the three states i,j, k: 



P-\ . . . 

 k I I 



P and R are now to be joined by our specifying what value 7?'s 

 parameter, call it a, is to take when P has any one of its states. 

 Suppose we decide on the relation Z (a transformation, single- 

 valued but not closed) : 



„ [state of P: \ i j k 

 ■ lvalue of a: ^ 2 3 2 



(The relation between P and a has been made somewhat irregular 

 to emphasise that the details are quite arbitrary and are completely 



4 49 



