AND ITS SELF-CONSERVATION. 123 



closest scrutiny of a given body, in its relation to space 

 merely, whether such body were moving or motionless. 

 Granted that space could be emptied of all objects with 

 the exception of one single body, that body could not be 

 said either to be at rest or to be in motion. For, space 

 being "infinite," in the double sense that it is without lim- 

 itation both externally and internally, there could be no 

 possible fixed point in space as such with reference to 

 which the body could be said to be either stationary or 

 moving. 



On the supposition, however, that two definite bodies 

 are in existence, it is evidently possible to recognize 

 whether the distance between the two remains the same, 

 or increases, or diminishes. And with the aid of the 

 spectroscope this would be possible, even though the ob- 

 servations were taken from one of the given bodies, 

 though, of course, on condition that the other body 

 should be incandescent. 



But, again, in such case it would be impossible to 

 judge whether the system composed by the two bodies 

 were moving or not, for the same reason that it would be 

 impossible to judge whether the single body in the former 

 case were moving or at rest. Nor would it be possible to 

 tell whether the one, or the other, or both the bodies 

 composing the system were moving, in case the distance 

 between them were ascertained to be increasing or dimin- 

 ishing. And, again, the two bodies might be revolving 

 about each other with any velocity, and the fact must re- 

 main forever unknown to an observer from either body, 

 supposing an axial rotation in each exactly corresponding 

 with the motion of their revolution about each other. 

 Or, supposing an axial rotation in the body from which 



