ON THE NOTION OF CAUSE 199 



circumstances, and would not have been true if the rest 

 of the universe had been different though subject to the 

 same laws. 



The essential function which causality has been sup- 

 posed to perform is the possibility of inferring the future 

 from the past, or, more generally, events at any time from 

 events at certain assigned times. Any system in which 

 such inference is possible may be called a " determin- 

 istic " system. We may define a deterministic system as 

 follows : 



A system is said to be " deterministic " when, given 

 certain data, e lt e 2 , . . ., C H , at times t lt t z , . . ., t H respec- 

 tively, concerning this system, if E, is the state of the 

 system at any time t, there is a functional relation of the 



form E^fie^^e,,^, ...,e n ,t n ,t}. (A) 



The system will be " deterministic throughout a given 

 period " if t, in the above formula, may be any time 

 within that period, though outside that period the 

 formula may be no longer true. If the universe, as a 

 whole, is such a system, determinism is true of the 

 universe ; if not, not. A system which is part of a deter- 

 ministic system I shall call " determined " ; one which is 

 not part of any such system I shall call " capricious." 



The events e v e z , . . ., e n I shall call " determinants " 

 of the system. It is to be observed that a system which 

 has one set of determinants will in general have many. 

 In the case of the motions of the planets, for example, 

 the configurations of the solar system at any two given 

 times will be determinants. 



We may take another illustration from the hypothesis 

 of psycho-physical parallelism. Let us assume, for the 

 purposes of this illustration, that to a given state of brain 



