76 THE RUNNING-DOWN OF THE UNIVERSE 



tion of nearly all physicists* that at the root of every- 

 thing there is a complete scheme of primary law govern- 

 ing the career of every particle or constituent of the 

 world with an iron determinism. This primary scheme 

 is all-sufficing, for, since it fixes the history of every 

 constituent of the world, it fixes the whole world- 

 history. 



But for all its completeness primary law does not 

 answer every question about Nature which we might 

 reasonably wish to put. Can a universe evolve back- 

 wards, i.e. develop in the opposite way to our own 

 system? Primary law, being indifferent to a time- 

 direction, replies, "Yes, it is not impossible". Secondary 

 law replies, "No, it is too improbable". The answers are 

 not really in conflict; but the first, though true, rather 

 misses the point. This is typical of some much more 

 commonplace queries. If I put this saucepan of water 

 on this fire, will the water boil? Primary law can answer 

 definitely if it is given the chance; but it must be 

 understood that "this" translated into mathematics 

 means a specification of the positions, motions, etc., of 

 some quadrillions of particles and elements of energy. 

 So in practice the question answered is not quite the 

 one that; is asked: If I put a saucepan resembling this 

 one in a few major respects on a fire, will the water 

 boil? Primary law replies, "It may boil; it may freeze; it 

 may do pretty well anything. The details given are insuf- 

 ficient to exclude any result as impossible." Secondary 

 law replies plainly, "It will boil because it is too im- 

 probable that it should do anything else." Secondary 

 law is not in conflict with primary law, nor can we re- 

 gard it as essential to complete a scheme of law already 



♦There are, however, others beside myself who have recently begun to 

 question it. 



