CAUSE AND EFFECT— PROBABILITY 133 



followed by the group we term combustion, has invariably 

 in our experience been accompanied by the sense-impres- 

 sion warmth. We may, if we are chemists, be able to 

 describe the chemical processes, the atomic changes or 

 motions to which the phenomenon of combustion has been 

 reduced ; we may, if we are physicists, describe the motion 

 of the ethereal medium, to which the phenomenon of 

 radiation of heat has been reduced ; we may, if we are 

 physiologists, be able to describe the nerve -motions by 

 aid of which the molecular motion of the finger-tips is 

 interpreted as the sense-impression warmth at the brain. 

 In all these cases we are dealing with the sequences of 

 various types of motion, into which we analyse or reduce 

 a variety of sense-impressions. Just as in the special case 

 of gravitation, we can also describe these sequences and 

 can frequently give a measure to the motions which we 

 conceive to take place, but we are still wholly unable to 

 state wJiy these motions occur. We may talk, if we 

 please, about the forces of combustion, the forces of radia- 

 tion, or even the forces inherent in nerve-substance ; we 

 might indeed say that the warmth, of which combustion 

 is the cause, is due to " an agglomeration of external or 

 internal forces," but in using such phrases we do not in- 

 troduce an iota of new knowledge, but too often a mountain 

 of obscurity. We hide the fact that all knowledge is 

 concise description, all cause is routine. 



Now it deserves special note that the sequences with 

 which we are dealing are all reducible to descriptions of 

 motion, or of change. We need not start arbitrarily with 

 the combustion of the coal ; its chemical constitution as an 

 element in the sequence of causes can, for example, be 

 carried back through a long past history in the evolution 

 of coal, and we cannot logically infer (p. 128) any begin- 

 ning or first cause in this sequence. Sequences of motion 

 or of change in natural phenomena go backwards and 

 forwards through an infinite range of causes, and to begin 

 or end them anywhere with a first or last cause is simply 

 to say that at such a point the sphere of knowledge ends 

 with an unthinkable x. The universe thus appears to the 



