DYNAMIC SYSTEMS 2/5 



on a dial. Temperature at a point can be specified numerically 

 and can be recorded on a dial. Pressure, angle, electric potential, 

 volume, velocity, torque, power, mass, viscosity, humidity, sur- 

 face tension, osmotic pressure, specific gravity, and time itself, 

 to mention only a few, can all be specified numerically and 

 recorded on dials. Eddington's statement on the subject is 

 explicit : ' The whole subject matter of exact science consists 

 of pointer readings and similar indications.' ' Whatever quan- 

 tity we say we are " observing ", the actual procedure nearly 

 always ends in reading the position of some kind of indicator on 

 a graduated scale or its equivalent.' 



Whether the restriction to dial-readings is justifiable with 

 living subjects will be discussed in S. 3/4. 



One minor point should be noticed as it will be needed later. 

 The absence of an entity can always be converted to a reading on 

 a scale simply by considering the entity to be present but in 

 zero degree. Thus, ' still air ' can be treated as a wind blowing at 

 m.p.h. ; c darkness ' can be treated as an illumination of foot- 

 candles ; and the giving of a drug can be represented by indicating 

 that its concentration in the tissues has risen from its usual value 

 of per cent. 



2/4. A system is any arbitrarily selected set of variables. It is 

 a list nominated by the experimenter, and is quite different from 

 the real ' machine '. 



At this stage no naturalness of association is implied, and the 

 selection is arbitrary. (' Naturalness ' is discussed in S. 2/14.) 



The variable ' time ' will always be used, so the dials will 

 always include a clock. But the status of ' time ' in the method 

 is unique, so it is better segregated. I therefore add the qualifi- 

 cation that ' time ' is not to be included among the variables of 

 a system. 



The Method 



2/5. It will be appreciated that every real i machine ' embodies 

 no less than an infinite number of variables, most of which must 

 of necessity be ignored. Thus if we were studying the swing of a 

 pendulum in relation to its length we would be interested in its 

 angular deviation at various times, but we would often ignore 



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