THE MACHINE WITH INPUT 4/11 



4/10. Ex. 4/7/4 has already shown that parts can, in general, be 

 coupled in different ways to form a whole. The defining of the 

 component parts does not determine the way of coupling. 



From this follows an important corollary. That a whole machine 

 should be built of parts of given behaviour is not sufficient to deter- 

 mine its behaviour as a whole: only when the details of coupling are 

 added does the whole's behaviour become determinate. 



FEEDBACK 



4/11. In S.4/7, P and R were joined so that P affected R while 

 R had no effect on P. P is said to dominate R, and (to anticipate 

 S.4/12) we may represent the relation between the parts by 



R 



(The arrow cannot be confused with that used to represent a transi- 

 tion (S.2/2), for the latter always relates two states, whereas the 

 arrow above relates two parts. In the diagrams to come, parts 

 will always be shown boxed.) 



Cybernetics is, however, specially interested in the case of S.4/8, 

 where each affects the other, a relation that may be represented by 



When this circularity of action exists between the parts of a dynamic 

 system, feedback may be said to be present. 



The definition of feedback just given is that most in accord with 

 the spirit of this book, which is concerned essentially with principles. 



Other definitions, however, are possible, and there has been some 

 dispute as to the best; so a few words in explanation may be useful. 

 There are two main points of view that have to be considered. 



On the one side stand those who are following the path taken by 

 this book — those whose aim is to get an understanding of the 

 principles behind the multitudinous special mechanisms that exhibit 

 them. To such workers, "feedback" exists between two parts when 

 each affects the other, as for instance, in 



A-' = 2xy 



-y^\ 



for >''s value affects how x will change and so does a's value affect y. 

 By contrast, feedback would not be said to be present in 



x' = 2x 



/ = x- 



53 



y 



