2/6 



DESIGN FOR A BRAIN 



the chemical composition of the bob, the reflecting power of its 

 surface, the electric conductivity of the suspending string, the 

 specific gravity of the bob, its shape, the age of the alloy, its 

 degree of bacterial contamination, and so on. The list of what 

 might be ignored could be extended indefinitely. Faced with 

 this infinite number of variables, the experimenter must, and of 

 course does, select a definite number for examination — in other 

 words, he defines his system. Thus, an experimenter once 

 drew up Table 2/5/1. He thereby defined a three- variable 



Table 2/5/1 



system, ready for testing. This experiment being finished, he later 

 drew up other tables which included new variables or omitted 

 old. By definition these new combinations were new systems. 



2/6. The variables being decided on, the recording apparatus 

 is now assumed to be attached to the ' machine ' and the experi- 

 menter ready to observe the dials. We must next specify what 

 power the experimenter has over the experimental situation. 



It is postulated that the experimenter can control any variable 

 he pleases : that he can make any variable take any arbitrary 

 value at any arbitrary time. The postulate specifies nothing 

 about the methods : it demands only that certain end-results 

 are to be available. In most cases the means to be used are 

 obvious enough. Take the example of S. 2/3 : an arbitrary 

 angular deviation of the pendulum can be enforced at any time 

 by direct manipulation ; an arbitrary angular momentum can be 

 enforced at any time by an appropriate impulse ; the cog can be 

 disconnected and shifted, the driving-weight wound up, the hand 

 moved, and the pendulum-bob lowered. 



By repeating the control from instant to instant, the experi- 

 menter can force a variable to take any prescribed series of 

 values. The postulate, therefore, implies that any variable can 

 be forced to follow a prescribed course. 



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