ON THE NOTION OF CAUSE 183 



terms as the argument to our propositional function. For 

 example, " if Socrates is a man, Socrates is mortal," is 

 necessary if Socrates is chosen as argument, but not if 

 man or mortal is chosen. Again, " if Socrates is a man, 

 Plato is mortal," will be necessary if either Socrates or 

 man is chosen as argument, but not if Plato or mortal is 

 chosen. However, this difficulty can be overcome by 

 specifying the constituent which is to be regarded as 

 argument, and we thus arrive at the following definition : 



" A proposition is necessary with respect to a given 

 constituent if it remains true when that constituent is 

 altered in any way compatible with the proposition re- 

 maining significant." 



We may now apply this definition to the definition of 

 causality quoted above. It is obvious that the argument 

 must be the time at which the earlier event occurs. Thus 

 an instance of causality will be such as : "If the event 

 e l occurs at the time t lt it will be followed by the event 

 e z ." This proposition is intended to be necessary with 

 respect to t lt i.e. to remain true however t l may be 

 varied. Causality, as a universal law, will then be the 

 following : " Given any event e 1} there is an event e z 

 such that, whenever e occurs, e z occurs later." But 

 before this can be considered precise, we must specify 

 how much later e z is to occur. Thus the principle be- 

 comes : 



" Given any event e lt there is an event e^ and a time- 

 interval T such that, whenever e^ occurs, e z follows after 

 an interval T ." 



I am not concerned as yet to consider whether this law 

 is true or false. For the present, I am merely concerned 

 to discover what the law of causality is supposed to be. 

 I pass, therefore, to the other definitions quoted above. 



