2/15 DESIGN FOR A BRAIN 



the subsequent trials merely confirming the first. The concept 

 of ' regularity ' thus conforms to the demand of S. 2/8 ; for it 

 is definable in terms of the field and is therefore wholly objective. 



The field of a regular system does not change with time. 



If, on testing, a system is found to be not regular, the experi- 

 menter is faced with the common problem of what to do with a 

 system that will not give reproducible results. Somehow he 

 must get regularity. The practical details vary from case to 

 case, but in principle the necessity is always the same : he must 

 try a new system. This means that new variables must be 

 added to the previous set, or, more rarely, some irrelevant variable 

 omitted. 



From now on we shall be concerned mostly with regular sys- 

 tems. We assume that preliminary investigations have been 

 completed and that we have found a system, based on the real 

 * machine ', that (1) includes the variables in which we are specially 

 interested, and (2) includes sufficient other variables to render 

 the whole system regular. 



2/15. For some purposes regularity of the system may be 

 sufficient, but more often a further demand is made before the 

 system is acceptable to the experimenter : it must be ' absolute \* 

 It will be convenient if I first define the concept, leaving the 

 discussion of its importance to the next section. 



If, on repeatedly applying primary operations to a system, it is 

 found that all the lines of behaviour which follow a state S are equal, 

 no matter how the system arrived at S, and if a similar equality 

 occurs after every other state S', S", . . . , then the system is 

 absolute. 



Consider, for instance, the two- variable system that gave the 

 two lines of behaviour shown in Table 2/15/1. 



On the first line of behaviour the state x = 0, y = 2-0 was 

 followed after 0-1 seconds by the state x = 0-2, y = 2-1. On 

 line 2 the state x = 0, y = 2-0 occurred again ; but after 0*1 

 seconds the state became x = 0-1, y = 1*8 and not x = 0-2, 

 y == 2*1. As the two lines of behaviour that follow the state 

 x = 0, y — 2-0 are not equal, the system is not absolute. 



A well-known example of an absolute system is given by the 



* (O.E.D.) Absolute : existent without relation to any other thing ; self- 

 sufficing ; disengaged from all interrupting causes. 



24 



