514 INDUCTION. 



is present in all the instances, no one of them indis- 

 pensable to the effect. On closer scrutiny, however, 

 it appears that though no one is always present, one 

 or other of several always is. If, on further analysis, 

 we can detect in these any common element, we may 

 be able to ascend from them to some one cause which 

 is the really operative circumstance in them all. Thus 

 it might, and perhaps will be, discovered, that in the 

 production of heat by friction, percussion, chemical 

 action, &c., the ultimate source is one and the same. 

 But if (as continually happens) we cannot take this 

 ulterior step, the different antecedents must be set 

 down as distinct causes, each sufficient of itself to 

 produce the effect. 



We may here close our remarks on the Plurality 

 of Causes, and proceed to the still more peculiar and 

 more complex case of the Intermixture of Effects, and 

 the interference of causes with one another : a case 

 constituting the principal part of the complication and 

 difficulty of the study of .nature ; and with which the 

 four only possible methods of directly inductive inves- 

 tigation by observation and experiment, are for the 

 most part, as will appear presently, quite unequal to 

 cope. The instrument of Deduction alone is adequate 

 to unravel the complexities proceeding from this 

 source ; and the four methods have little more in theic 

 power than to supply premisses for our deductions. 



4. A concurrence of two or more causes, not 

 separately producing each its own effect, but interfer- 

 ing with or modifying the effects of one another, takes 

 place, as has already been explained, in two different 

 ways. In the one case, which is exemplified by the 

 joint operation of different forces in mechanics, the sepa- 

 rate effects of all the causes continue to be produced, 



