28 HOLISM AND EVOLUTION chap. 



but always in both together, and where objects are not ob- 

 served by themselves, but always as elements or items in the 

 stream of perceived events. Nay more, it can be easily shown 

 that the very ideas of Space and Time interpenetrate each 

 other and are dependent on each other. Succession or the 

 time-series, and co-existence or the space-series, are neces- 

 sary to each other and would not be even intelligible apart 

 from each other. For the succession (time) would perish at 

 each step and would not even form a series, unless it had en- 

 duringness or co-existence (space). And similarly the co- 

 existence (space) would stop at its first step and would not be 

 spread out or extended unless it had also succession (time). 



To Einstein this concept of a Space-Time continuum 

 proved most welcome and fruitful, and he proceeded to 

 apply it to the explanation of all movements in the universe, 

 not only to uniform and rectilinear motions which take place 

 in uniform Euclidean space, but also to rotating and accele- 

 rated motions which take place in a gravitational field of a 

 non-Euclidean character. 



His first step was to illustrate, by purely theoretical 

 considerations, the fact that a body under the influence of a 

 constant force, and therefore moving with a constantly in- 

 creasing acceleration, would to an observer situated on it 

 behave in exactly the same way as a body acted on by 

 gravitation. Thus suppose a man enclosed in a cage so that 

 he cannot observe any other body and cannot notice his own 

 motion. And suppose this cage suspended in distant space 

 where there is no gravitation. And suppose further that 

 this cage is drawn upward with a constant force, so that it 

 moves faster and faster with a constantly increasing accel- 

 eration. In truth the case is therefore one of acceleration. 

 But by the enclosed observer this motion and acceleration of 

 the cage and himself will not be noticed. He would only feel 

 like being pulled down by his own weight. If he loses any 

 object from his hand, it will fall to the bottom of the cage 

 like a stone dropped on the earth. What is more, the rate 

 at which the object drops is the same, whatever its figure or 

 size or amount of material. The fact italicised is distinctive 



