ON THE NOTION OF CAUSE 213 



causality which is used in science differs from that of 

 common sense and traditional philosophy ; V., what new 

 light is thrown on the question of free will by our 

 analysis of the notion of "cause." 



I. By a "causal law' I mean any general proposition 

 in virtue of which it is possible to infer the existence of 

 one thing or event from the existence of another or of 

 a number of others. If you hear thunder without having 

 seen lightning, you infer that there nevertheless was a 

 flash, because of the general proposition, " All thunder is 

 preceded by lightning." When Robinson Crusoe sees a 

 footprint, he infers a human being, and he might justify 

 his inference by the general proposition, " All marks in 

 the ground shaped like a human foot are subsequent to 

 a human being's standing where the marks are." When 

 we see the sun set, we expect that it will rise again the 

 next day. When we hear a man speaking, we infer that 

 he has certain thoughts. All these inferences are due to 

 causal laws. 



A causal law, we said, allows us to infer the existence 

 of one thing (or event) from the existence of one or more 

 others. The word " thing ' here is to be understood as 

 only applying to particulars, i.e. as excluding such logical 

 objects as numbers or classes or abstract properties and 

 relations, and including sense-data, with whatever is 

 logically of the same type as sense-data. 1 In so far as a 

 causal law is directly verifiable, the thing inferred and the 

 thing from which it is inferred must both be data, 

 though they need not both be data at the same time. 

 In fact, a causal law which is being used to extend our 

 knowledge of existence must be applied to what, at the 



1 Thus we are not using " thing " here in the sense of a class of cor- 

 related "aspects," as we did in Lecture III. Each "aspect" will count 

 separately in stating causal laws. 



