CHAPTER 6 



Parameters 



6/1. So far, we have discussed the changes shown by the vari- 

 ables of an absolute system, and have ignored the fact that all its 

 changes occur on a background, or on a foundation, of constancies. 

 Thus, a particular simple pendulum provides two variables which 

 are known (S. 2/15) to be such that, if we are given a particular 

 state of the system, we can predict correctly its ensuing be- 

 haviour ; what has not been stated explicitly is that this is true 

 only if the length of the string remains constant. The background, 

 and these constancies, must now be considered. 



Every absolute system is formed by selecting some variables out 

 of the totality of possible variables. ' Forming a system ' 

 means dividing all possible variables into two classes : those 

 within the system and those without. These two types of variable 

 are in no way different in their intrinsic physical nature, but they 

 stand in very different relations to the system. 



6/2. Given a system, a variable not included in it will be 

 described as a parameter. The word variable will, from now on, 

 be reserved for one within the system. 



In general, given a system, the parameters will differ in their 

 closeness of relation to it. Some will have a direct relation to it : 

 their change of value would affect the system to a major degree ; 

 such is the parameter ' length of pendulum ' in its relation to the 

 two-variable system of the previous section. Some are less 

 closely related to it, their changes producing only a slight effect 

 on it ; such is the parameter ' viscosity of the air ' in relation to 

 the same system. And finally, for completeness, may be men- 

 tioned the infinite number of parameters that are without detect- 

 able effect on the system ; such are the brightness of the light 

 shining on the pendulum, the events in an adjacent room, and the 

 events in the distant nebulae. Those without detectable effect 



72 



