TIME'S ARROW 69 



recognise a directed time; he is shocked that the new 

 picture should expose this so glaringly. 



Without any mystic appeal to consciousness it is 

 possible to find a direction of time on the four-dimen- 

 sional map by a study of organisation. Let us draw an 

 arrow arbitrarily. If as we follow the arrow we find 

 more and more of the random element in the state of 

 the world, then the arrow is pointing towards the future; 

 if the random element decreases the arrow points 

 towards the past. That is the only distinction known to 

 physics. This follows at once if our fundamental con- 

 tention is admitted that the introduction of randomness 

 is the only thing which cannot be undone. 



I shall use the phrase "time's arrow" to express this 

 one-way property of time which has no analogue in 

 space. It is a singularly interesting property from a 

 philosophical standpoint. We must note that — 



( 1 ) It is vividly recognised by consciousness. 



(2) It is equally insisted on by our reasoning faculty, 

 which tells us that a reversal of the arrow would render 

 the external world nonsensical. 



(3) It makes no appearance in physical science except 

 in the study of organisation of a number of individuals. 

 Here the arrow indicates the direction of progressive 

 increase of the random element. 



Let us now consider in detail how a random element 

 brings the irrevocable into the world. When a stone 

 falls it acquires kinetic energy, and the amount of the 

 energy is just that which would be required to lift the 

 stone back to its original height. By suitable arrange- 

 ments the kinetic energy can be made to perform this 

 task; for example, if the stone is tied to a string it can 

 alternately fall and reascend like a pendulum. But if 

 the stone hits an obstacle its kinetic energy is converted 



