68 SCIENCE AND METHOD. 



chance. If we knew exactly the laws of nature and 

 the situation of the universe at the initial moment, 

 we could predict exactly the situation of that same 

 universe at a succeeding moment. But, even if it 

 were the case that the natural laws had no longer 

 any secret for us, we could still only know the initial 

 situation approximately. If that enabled us to predict 

 the succeeding situation with the same approximation, 

 that is all we require, and we should say that the 

 phenomenon had been predicted, that it is governed 

 by laws. But it is not always so ; it may happen that 

 small differences in the initial conditions produce very 

 great ones in the final phenomena. A small error in 

 the former will produce an enormous error in the 

 latter. Prediction becomes impossible, and we have 

 the fortuitous phenomenon. 



Our second example will be very much like our 

 first, and we will borrow it from meteorology. Why 

 have meteorologists such difficulty in predicting the 

 weather with any certainty ? Why is it that showers 

 and even storms seem to come by chance, so that 

 many people think it quite natural to pray for rain 

 or fine weather, though they would consider it 

 ridiculous to ask for an eclipse by prayer ? We see 

 that great disturbances are generally produced in 

 regions where the atmosphere is in unstable equilib- 

 rium. The meteorologists see very well that the 

 equilibrium is unstable, that a cyclone will be formed 

 somewhere, but exactly where they are not in a 

 position to say ; a tenth of a degree more or less at 

 any given point, and the cyclone will burst here and 

 not there, and extend its ravages over districts it 

 would otherwise have spared. If they had been aware 



