288 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



equations predict future events, they predict past ones as 

 well. Time in mechanics is then completely reversible or 

 two-way time." * Eddington expresses the same fact by 

 saying that the laws of nature are indifferent as to the doing 

 or the undoing of an event, hence "they must be indifferent 

 as to a direction of time from past to future. That is their 

 common feature, and it is seen at once when (as usual) 

 the laws are formulated mathematically. There is no more 

 distinction between past and future than between right and 

 left. In algebraic symbolism, left is — x, right is + x\ 

 past is — t, future is + £." 2 The laws of mechanics enable 

 us to compute the time-difference between one event and 

 another, but they do not assign an absolute value to this 

 difference and hence do not enable us to determine which 

 event is earlier and which is later. 3 



The question hinges upon the meaning of reversible proc- 

 esses. "All processes, the effects of which can be completely 

 annulled, are called reversible. Irreversible processes are 

 such that, with the use of all possible physical means, a 

 complete restoration of the initial state everywhere cannot 

 be achieved." 4 The problem, then, is to determine the 

 actual character of physical processes. Are all reversible, 

 all irreversible, or some reversible and some not? In the 

 first case the laws should contain a two-way time, in the 

 second case a directional time, and in the third case distinct 

 laws for the two types of process would be required. But 

 when the problem is formulated in this way the solution 

 is readily seen to be quite arbitrary. Whether nature is 

 considered to be reversible or irreversible depends upon the 

 level of abstraction at which one looks at nature. At the em- 

 pirical level, as will be seen in the next section, time pos- 

 sesses an obvious directional feature, and any accurate 

 description of physical processes must portray this fact. 

 But if one neglects sufficient of the empirical features, 

 e.g., friction and heat conduction, any physical process 



1 R. B. Lindsay and H. Margenau, Foundations of Physics, p. 76. 



2 Nature of the Physical World, p. 66. 



3 Ibid., p. 296. * R. B. Lindsay and H. Margenau, op. cit., p. 196 



