254 INDUCTION. 



it does occur, our methods of induction ought to be capable of ascer- 

 taining and establishing. For this, however, there is required no 

 peculiar method. When an effect is really producible by two or more 

 causes, the process for detecting them is in no way different from that 

 by which we discover single causes. They may (first) be discovered 

 as separate sequences, by separate sets of instances. One set of ob- 

 Bervationfj or experiments shows that the sun is a cause of heat, another 

 that friction is a source of it, another tliat percussion, another that elec- 

 tricity, another that chemical action is such a source. Or (secondly) 

 the plurality may come to light in the course of collating a number of 

 instances, when we attempt to find some cu-cumstance in which they 

 all agi'ee, and fail in doing so. We find it impossible to trace, in all 

 the cases in which the effect is met with, any common circiuustance. 

 We find that we can eliminate all the antecedents ; that no one of them 

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

 ysis, 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 cir- 

 cumstance in them all. Thus it might, andperhaps 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 hap- 

 pens) 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 pro- 

 ceed to the still more peculiar and more complex case of the Intermix- 

 ture 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 investigation by observation and experiment, are 

 for tJre most pait, as will appear presently, quite unequal to cope. 

 The instrument of Deduction alone is adequate to unravel the com- 

 plexities proceeding from this source ; and the foiu' methods have httle 

 more in their power than t© supply premisses for our deductions. 



§ 4. A concuiTence of two or more causes, not separately producing 

 each its own effect, but intei^fering 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 separate effects of all the causes con- 

 tinue to be produced, but are compounded with one another, and dis- 

 apfjear in one total, ' In the other case, illustrated by the case of chem- 

 ical action, the separate effects cease entirely, and are succeeded by 

 phenomena altogether different, and governed by different laws. 



Of these cases the former is by far the more frequent, and this case 

 it is which, for the most part, eludes the grasp of our experimental 

 methods. The other and exceptional case is essentially amenable to 

 them. When the laws of the original agents cease entirely, and a 

 phenomenon makes its appearance, which, with reference to those 

 laws, is quite heterogeneous ; when, for example, two gaseous sub- 

 stances, hydrogen and oxygen, on being brought together, throw off 

 their peculiar properties, and produjce the substance called water ; in 

 such cases the new fact may be subjected to experimental inquiry, like 



