DESIGN FOR A BRAIN 



12/18 



over an interval in which all variables but one are constant, the 

 corresponding line of behaviour must run as a straight line parallel 

 to the axis of the variable that is changing. If all but two are 



inactive (along some line of behaviour), 

 that line in the phase-space may curve 

 but it must remain in the two-dimen- 

 sional plane parallel to the two corre- 

 sponding axes; and so on. If all the 

 variables are constant, the line 

 naturally becomes a point — at the 

 state of equilibrium. Thus a three- 

 variable system might give the line of 

 behaviour shown in Figure 12/18/1. 



In the interval before they reach 

 equilibrium, such variables will, of 

 course, behave as part-functions. 

 Through the remaining chapters they 

 will show their importance. For convenience of description, a 

 part-function (described in time by a variable) will be said to be 

 active or inactive (at a given point on a line of behaviour) 

 according to whether the variable is changing or remaining 

 constant. 



Figure 12/18/1. In the dif- 

 ferent stages the active 

 variables are : A, y ; B, y 

 and 2 ; C,z; D x ; E y ; 

 F x and z. 



170 



