152 THE LIMITATIONS OF SCIENCE 



of light from the sun to the earth, as if we were 

 considering a real mechanical motion of a ponderable 

 body, although in the intervening space there is no 

 matter. The only thing we have been able to observe 

 is, that light emitted from the sun appears on the earth 

 some time later. Light, as a phenomenon, does not 

 exist unless it is associated with matter; we can no 

 more discuss the amount of light or its velocity in 

 vacuous space than we can speak of the temperature 

 of such space. Light has a true mechanical velocity 

 when passing through space occupied by matter, for 

 then we can observe and measure its path as well as 

 the time function and so obtain a value for velocity. 

 From observations on the velocity of light in matter 

 and from analogy to the phenomena of sound trans- 

 mission, which takes place only in spaces occupied by 

 matter, we by the hypothetical method transfer the 

 measurements and laws of light in transparent bodies 

 to space not occupied by matter. The very assump- 

 tion of a light velocity in immaterial spaces requires 

 us also tacitly to assume that something, in a mechan- 

 ical sense, is moving. Once we have granted that light 

 is something moving, then all the other phenomena of 

 light permit of a hypothetical mechanical explanation, 

 and we have the right to speak of the momentum and 

 energy of this light something, whether it be corpuscles 

 or waves. But it is the easiest thing in the world to 

 forget that we can never obtain any real knowledge 



