CYCLIC METHOD OF PHYSICS 261 



what would really be involved in a complete explanation 

 of anything physical. 



Einstein's law in its analytical form is a statement that 

 in empty space certain quantities called potentials obey 

 certain lengthy differential equations. We make a 

 memorandum of the word ''potential" to remind us 

 that we must later on explain what it means. We might 

 conceive a world in which the potentials at every moment 

 and every place had quite arbitrary values. The actual 

 world is not so unlimited, the potentials being restricted 

 to those values which conform to Einstein's equations. 

 The next question is, What are potentials? They can 

 be defined as quantities derived by quite simple mathe- 

 matical calculations from certain fundamental quantities 

 called intervals. (Mem. Explain "interval".) If we 

 know the values of the various intervals throughout the 

 world definite rules can be given for deriving the values 

 of the potentials. What are intervals? They are rela- 

 tions between pairs of events which can be measured with 

 a scale or a clock or with both. (Mem. Explain "scale" 

 and "clock".) Instructions can be given for the correct 

 use of the scale and clock so that the interval is given by 

 a prescribed combination of their readings. What are 

 scales and clocks? A scale is a graduated strip of mat- 

 ter which. . . . (Mem. Explain "matter".) On second 

 thoughts I will leave the rest of the description as "an 

 exercise to the reader" since \t would take rather a long 

 time to enumerate all the properties and niceties of 

 behaviour of the material standard which a physicist 

 would accept as a perfect scale or a perfect clock. We 

 pass on to the next question, What is matter? We have 

 dismissed the metaphysical conception of substance. We 

 might perhaps here describe the atomic and electrical 

 structure of matter, but that leads to the microscopic 



