IS THE ORGANISM A MECHANISM? 653 



that all the molecules should collide end on, so leading to a 

 reversal of the physical history of the gas. The calculation has 

 been made by Boltzmann. It is possible that this simultaneous 

 reversal of motion of all the molecules in a decilitre of perfect 

 gas may occur, but in order to witness it we might have to 

 observe the gas for a lapse of time represented (in centuries) by 

 unity followed by a thousand millions of ciphers ! The chance 

 is thus very small. It is about equal to the chance that all the 

 houses in London might catch fire, independently of each other, 

 on the same day, or that all the men in London might commit 

 suicide, independently of each other, on the same day. An 

 insurance office would certainly disregard such risks. They 

 would say that it was " practically impossible." So we must 

 say that it is " practically impossible " that heat should flow, of 

 itself, from a colder to a hotter body ; that perpetual motion is a 

 practical impossibility; and that it is also practically impossible 

 that the second law of thermodynamics does not apply to all 

 physical and chemical transformations. We say, therefore, 

 that all natural phenomena have the same tendency — towards 

 degradation of energy, or augmentation of entropy — and much 

 of our power of so ordering naturally occurring events, or of 

 forecasting future events, depends on our confident assumption 

 that this tendency will continue to hold true. It is almost certain 

 that the second law of thermodynamics will be valid in all our 

 future experience as it has been valid in all our past. 



" Almost certain," we say, but not logically capable of demon- 

 stration — not inevitable. The first law — that of conservation — 

 is a priori certain, that is, it is unthinkable that it should not be 

 universally true. But the second law is only a probability— a 

 very great probability if we like. Now let us regard the decilitre 

 of gas of our example as a " model," in a kind of way, of the 

 universe. Like the universe we may describe it as a " collection 

 of isolated mass points, devoid of rotatory inertia, moving in 

 accordance with Newton's laws, and attracting or repelling each 

 other with forces which are continuous or discontinuous functions 

 of the distance between them." If, then, we extend the general 

 conclusions deduciblefrom the kinetic gas-theory to the universe 

 itself, we might consider whether heat may flow, of itself, from 

 colder to hotter regions — in general, whether there may not be, 

 somewhere, a restoration of available energy. The possible form 

 of such a speculative process would depend on the cosmology 

 43 



