124 THE WOKLD-EKERGY 



observations are taken, and not corresponding with any 

 given movement which the bodies might have about one 

 another, the observer could never detect the axial rotation 

 in his own sphere. On the contrary, he must inevitably 

 attribute to the observed body a movement about his own 

 sphere, even though the bodies were at rest with reference 

 to each other. While, in case an actual revolution exr 

 isted, it could in no way be detected, and the apparent 

 motion might be exactly opposite to the actual one. The 

 former case is sufficiently illustrated by the apparent revo- 

 lution of the sun around the earth; the latter by the 

 apparent 'motion of the moon contrary to its actual 

 motion about our planet discrepancies which could never 

 have been discovered save through observation of the 

 motion of many bodies. 



Finally, what has been said of the relativity of motion 

 must be true in any system composed of any number of 

 bodies. Any motion of the system as a whole could 

 never be detected, save in comparison with some body, or 

 group of bodies, outside the system. That is, no positive 

 judgment could ever be formed of any state of motion or ' 

 rest respecting the bodies composing the system, save 

 with reference to one another. 



Thus, we may perhaps be permitted to say, we 

 know absolutely that all our knowledge of the mo- 

 tions of bodies must be relative though the special 

 discussions of those motions constitute several of the 

 most important of the " exact sciences" which, as such, 

 ought, it would seem, to lead us to absolute knowl- 

 edge of some sort. Perhaps, after all, it will yet be 

 discovered that these are the absolute sciences of the 

 relative. 



