CLASSICAL AND QUANTUM LAWS 197 



infinitesimal steps, there is no essential difference in 

 my mode of progress on the two days. And so it makes 

 no difference whether the electron steps from one orbit 

 to the next lower or comes down in a spiral when the 

 number of steps is innumerably great. The succession 

 of lumps of energy cast overboard merges into a con- 

 tinuous outflow. If you had the formulae before you, 

 you would find that the period of the light and the 

 strength of radiation are the same whether calculated by 

 the Monday or the Tuesday method — but only when 

 the quantum number is infinitely great. The disagree- 

 ment is not very serious when the number is moderately 

 large; but for small quantum numbers the atom cannot 

 sit on the fence. It has to decide between Monday 

 (classical) and Tuesday (quantum) rules. It chooses 

 Tuesday rules. 



If, as we believe, this example is typical, it indicates 

 one direction which the reconstruction of ideas must 

 take. We must not try to build up from classical con- 

 ceptions, because the classical laws only become true and 

 the conceptions concerned in them only become defined 

 in the limiting case when the quantum numbers of the 

 system are very large. We must start from new con- 

 ceptions appropriate to low as well as to high numbered 

 states; out of these the classical conceptions should 

 emerge, first indistinctly, then definitely, as the number 

 of the state increases, and the classical laws become 

 more and more nearly true. " I cannot foretell the result 

 of this remodelling, but presumably room must be 

 found for a conception of "states", the unity of a 

 state replacing the kind of tie expressed by classical 

 forces. For low numbered states the current vocabulary 

 of physics is inappropriate; at the moment we can 

 scarcely avoid using it, but the present contradictoriness 



