CHAPTER 14 



Repetitive Stimuli and Habituation 



14/1. This chapter continues the study of the polystable system 

 but is something of a digression; and the reader who proceeds 

 directly to Chapter 15 will lose nothing of the logical thread. 

 Nevertheless, it has been included for two reasons. 



The first is that it will give us practice in understanding the 

 polystable system, and will show how systems of that type can 

 be discussed in terms that are both general and precise. 



A second reason is that it gives another example of the thesis 

 that pervades the book : — -When a system ' runs to equilibrium ' 

 one's first impression is that what is interesting has now come to 

 an end — an impression that is often valid when the system is 

 simple, and the equilibrium that of a run-down clock. What has 

 been largely overlooked, and what this book attempts to display, 

 is that when a complex system runs to equilibrium, the equilibrium 

 necessarily implies a complex relationship between the states of 

 the various parts. When the relationship between the states of 

 the parts is examined, they will show unusual and striking 

 features, features that are of peculiar interest to the student of 

 behaviour. Thus Chapter 8, on the Homeostat, showed ' only ' a 

 system running to equilibrium (e.g. Figure 8/5/1); yet because the 

 system and conditions were structured, interesting relations could 

 be traced between the actions and interactions of the various 

 parts at and around the terminal equilibrium. These relations 

 are what we identified in Chapter 5 as ' adaptation '. The present 

 chapter will give another example of how a system, ' merely ' 

 running to equilibrium under a complex repetitive input, also 

 produces behaviour of psychological and physiological interest. 



14/2. First a definition. When there are many states of equili- 

 brium in a field, then as every line of behaviour must end at some 

 state of equilibrium (or cycle), the lines of behaviour collect into 

 sets, such that the lines in one set come to one common termina- 



184 



