CLASSICAL AND NEW MECHANICS 149 



one fact that they inclose this space. By deduction 

 and by experience I am convinced that this space is 

 not empty, but contains air. But it is really more 

 difficult to appreciate the existence of the air, if it be 

 at rest, than its non-existence. This belief is borne 

 out by the slow advance in the conception of the exist- 

 ence of gases. And it also seems true to me that I 

 have an adequate idea of an area inclosed in a tri- 

 angle, because I can think of the area inclosed by three 

 abstract lines and refrain from thinking of the con- 

 crete volume of any real lines drawn with a pencil. 



As for the concept time, we have a twofold sense 

 of it; one is the coincidence of an event with the posi- 

 tion of the hands of a clock or of the earth. But we 

 have, in addition, a much more general idea of time, 

 as mere succession of events without any reference to 

 such a standard of measurement as the second or 

 minute. This might be called our belief that events 

 entirely unconnected with our own experience occur 

 successively just as those events do which we com- 

 pare with a clock. We have, on awaking from sleep, 

 a distinct and clear idea that events have transpired 

 successively during our unconsciousness and without 

 any reference to a measured interval of time. This 

 general knowledge of " before and after " is appa- 

 rently possible to an animal ; a dog will with certainty 

 expect a reward after the performance of a trick and 

 not before or during it. 



