PRIMARY AND SECONDARY LAW] 75 



improbable; but the improbability is vague and does 

 not confront us as a paralysing array of figures, whereas 

 the chance against a breach of the second law (i.e. 

 against a decrease of the random element) can be stated 

 in figures which are overwhelming. 



I wish I could convey to you the amazing power of 

 this conception of entropy in scientific research. From 

 the property that entropy must always increase, practical 

 methods of measuring it have been found. The chain 

 of deductions from this simple law have been almost 

 illimitable; and it has been equally successful in con- 

 nection with the most recondite problems of theoretical 

 physics and the practical tasks of the engineer. Its 

 special feature is that the conclusions are independent 

 of the nature of the microscopical processes that are 

 going on. It is not concerned with the nature of the 

 individual; it is interested in him only as a component 

 of a crowd. Therefore the method is applicable in 

 fields of research where our ignorance has scarcely begun 

 to lift, and we have no hesitation in applying it to prob- 

 lems of the quantum theory, although the mechanism 

 of the individual quantum process is unknown and at 

 present unimaginable. 



Primary and Secondary Law. I have called the laws 

 controlling the behaviour of single individuals "primary 

 laws", implying that the second law of thermodynamics, 

 although a recognised law of Nature, is in some sense a 

 secondary law. This distinction can now be placed on 

 a regular footing. Some things never happen in the 

 physical world because they are impossible; others 

 because they are too improbable. The laws which forbid 

 the first are the primary laws; the laws which forbid the 

 second are the secondary laws. It has been the convic- 



