SPACE, TIME 289 



can be described by laws which are invariant under the 

 substitution of + t for — t. To take a particular instance, 

 the motions of the planets can be described by reversible 

 laws if one chooses to ignore tidal friction, but such a law 

 would be more abstract than one which includes reference 

 to this important fact. Hence a classification of natural 

 processes into reversible (movements of perfectly rigid bodies 

 and of incompressible fluids, oscillations of a simple pendu- 

 lum, movements of the planets) and irreversible (heat con- 

 duction, heat generation by friction, diffusion, explosions), 

 such as that given by Lindsay and Margenau, 1 is based 

 upon a difference of degree rather than one of kind. In the 

 former the temporal aspects seem unimportant, and we feel 

 justified in neglecting them, while in the latter the temporal 

 aspects seem important, and we recognize the artificiality 

 which results in our symbolic scheme if we fail to include 

 them. The whole issue, in fact, is essentially one of con- 

 venience. "We can at any rate safely say that, in the actual 

 use of the time concept in physics, the physicist will consult 

 his convenience with respect to reversibility or irreversi- 

 bility. The situation here is precisely that encountered in 

 connection with other concepts. If the demands for clarity 

 and simplicity cannot be met by the more intuitive notions, 

 the physicist has no hesitation in modifying these notions 

 and subliming them into more abstract concepts.' 



»? 



EMPIRICAL SPACE AND TIME 



Empirical space and time may be described as relative, 

 plural, discrete, finite, non-homogeneous, and anisotropic. Cor- 

 responding remarks may be made with reference to each 

 of these features. 



Relativity. By the relativity of space and time is meant 

 their inseparability from events. Space and time, from the 

 empirical point of view, are simply features of happenings 

 in space and time. At this level there is no empty space, 

 and there is no unoccupied time. 



1 Ibid., p. 197. 2 Ibid., p. 76. 



