ml vi.] CAUSATION. 151 



on a frosty morning, or by a piece of good news, or by a 

 rhythmical increase in the rate of nutrition for which no 

 definite external cause is assignable. It is the business of 

 induction to eliminate, as far as possible, all these coexistent 

 "possible causes, so as to ascertain, after the elimination, 

 whether the sequence between the presumed cause and the 

 effect is invariable. If it turns out to be so, and, still better, 

 if by reasoning deductively from the experimentally-ascer- 

 tained action of the coffee or the medicine upon the organic 

 tissues involved in the case, further proof of the invariable- 

 ness of the given sequences can be obtained, — then we say 

 that we have detected a case of true causation. When we 

 have extended our inquiries in any case so far as to be able 

 to predicate invariable sequence, then we predicate causation. 

 A moment's reflection, however, will show us that there 

 are sequences which have been invariable throughout the 

 whole course of human experience, but which are not re- 

 garded as causal sequences. Ever since there have been 

 conscious minds to interpret phenomena, day has followed 

 night, and night has followed day, and yet no one would say 

 that day causes night, or that night causes day. In order 

 to include such cases as this, we must limit still further 

 our definition of causation. The sequence must be uncon- 

 ditional as well as invariable. This, as Mr. Mill observes, 

 "is what writers mean when they say that the notion of 

 cause involves the idea of necessity. If there be any mean- 

 ing which confessedly belongs to the term ' necessity,' it is 

 unconditionalness. 1 That which is necessary, that which must 

 be, means that which will be, whatever supposition wo may 

 make in regard to all other things. The succession of day 

 and night evidently is not necessary in this sense. It is 

 conditional on the occurrence of other antecedents. That 

 which will be followed by a given consequent when, and 



1 This, it will be seen, agrees with Mr. Lewes's admirable view of Nece*> 

 lity, cited above in Chapter III. 



