PLURALITY OF CAUSES. 311 



est scientific acquirements argue as pitiably as the merest ignoramus. For 

 though they have made many sound inductions, they have not learned from 

 them (and Dr. Whewell thinks there is no necessity that they should learn) 

 the principles of inductive evidence. 



CHAPTER X. 



OF PLURALITY OP CAUSES, AND OF THE INTERMIXTURE OF EFFECTS. 



§ 1. In the preceding exposition of the four methods of observation 

 and experiment, by which we contrive to distinguish among a mass of co- 

 existent phenomena the particular effect due to a given cause, or the par- 

 ticular cause which gave birth to a given effect, it has been necessary to 

 suppose, in the first instance, for the sake of simplification, that this ana- 

 lytical operation is encumbered by no other diflSculties than what are essen- 

 tially inherent in its nature ; and to represent to ourselves, therefore, every 

 effect, on the one hand as connected exclusively with a single cause, and 

 on the other hand as incapable of being mixed and confounded with any 

 other co-existent effect. We have regarded abcde, the aggregate of 

 the phenomena existing at any moment, as consisting of dissimilar facts, 

 a, b, c, d, and e, for each of which one, and only one, cause needs be sought; 

 the difiiculty being only that of singling out this one cause from the mul- 

 titude of antecedent circumstances, A,B,C,D, and E. The cause indeed 

 may not be simple ; it may consist of an assemblage of conditions ; but we 

 have supposed that there was only one possible assemblage of conditions 

 from which the given effect could result. 



If such were the fact, it would be comparatively an easy task to investi- 

 gate the laws of nature. But the supposition does not hold in either of 

 its parts. In the first place, it is not true that the same phenomenon is 

 always produced by the same cause : the effect a may sometimes arise 

 fi'om A, sometimes from B. And, secondly, the effects of different causes 

 are often not dissimilar, but homogeneous, and marked out by no assign- 

 able boundaries from one another : A and 13 may produce not a and b, but 

 different portions of an effect a. The obscurity and difficulty of the inves- 

 tigation of the laws of phenomena is singularly increased by the necessi- 

 ty of adverting to these two circumstances : Intermixture of Effects, and 

 Plurality of Causes. To the latter, being the simpler of the two considera- 

 tions, we shall first direct our attention. 



It is not true, then, that one effect must be connected with only one 

 cause, or assemblage of conditions; that each phenomenon can be pro- 

 duced only in one way. There are often several independent modes in 

 which the same phenomenon could have originated. One fact may be the 

 consequent in several invariable sequences; it may follow, with equal uni- 

 formity, any one of several antecedents, or collections of antecedents. 

 Many causes may produce mechanical motion; many causes may produce 

 some kinds of sensation ; many causes may produce death. A given effect 

 may really be produced by a certain cause, and yet be perfectly capable of 

 being produced without it. 



§ 2. One of the principal consequences of this fact of Plurality of Causes 

 is, to render the first of the inductive methods, that of Agreement, uncer- 



