296 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



which movement is reversible space may be considered 

 isotropic. 



If time is anisotropic, it can be so only in the directional 

 sense, for time is one-dimensional. In this sense there seems 

 little reason to doubt the fundamental anisotropy of time. 

 Time has a definite direction, or arrow; it passes from the 

 future into the past, and not from the past into the future. 

 The directional character of time is intimately connected 

 with one's conception of causal influences, hence a reversal 

 of time would be essentially unthinkable. A life in which 

 one first dies, then is married, and finally is born, could 

 hardly have any kinship with the present mode of being. 

 To be sure, this is a series of events, not time itself. Yet from 

 the empirical point of view time is inseparable from change 

 of events, and it is because changes of one kind always 

 occur, and changes of another kind never occur that a 

 reversibility of time becomes unthinkable. It seems hard to 

 doubt that an awareness, more or less vague, of the going-on 

 of time is present at all levels of consciousness. Eddington 

 identifies this basic recognition of time's arrow with one's 

 feeling of "becoming." * The notion of time as something 

 which "flows," or "passes" is essential to it at the empirical 

 level; but "flow' and "passage' are both directional in 

 character. Hence time must be described as asymmetrical, 

 or irreversible. 



SPACE AND TIME: OPERATIONAL DERIVATION 



The analysis of space and time has thus far revealed an 

 important fact. There are at least two conceptions of space 

 and two conceptions of time, which are sharply to be dif- 

 ferentiated from one another. In general the properties 

 possessed by the one are contradicted by those possessed by 

 the other. Immediately the question arises as to which is 

 more adequate as a description of the space and time of 

 nature, i.e., which is real space and which is real time. 

 Although this formulation of the problem is essentially 



1 Nature of the Physical World, p. 69. 



