INTERMIXTURE OF EFFECTS. 495 



the separate causes do not terminate and give place to others, 

 thereby ceasing to form any part of the phenomenon to be 

 investigated ; on the contrary, they still take place, but are 

 intermingled with, and disguised by, the homogeneous and 

 closely allied effects of other causes. They are no longer 

 a, b, c, d, e, existing side by side, and continuing to be sepa 

 rately discernible; they are + a, a, ^ b, b, 2 b, &c., some 

 of which cancel one another, while many others do not appear 

 distinguishably, but merge in one sum : forming altogether 

 a result, between which and the causes whereby it was pro 

 duced there is often an insurmountable difficulty in tracing by 

 observation any fixed relation whatever. 



The general idea of the Composition of Causes has been 

 seen to be, that though two or more laws interfere with one 

 another, and apparently frustrate or modify one another s 

 operation, yet in reality all are fulfilled, the collective effect 

 being the exact sum of the effects of the causes taken sepa 

 rately. A familiar instance is that of a body kept in equili 

 brium by two equal and contrary forces. One of the forces 

 if acting alone would carry the body in a given time a certain 

 distance to the west, the other if acting alone would carry it 

 exactly as far towards the east ; and the result is the same as 

 if it had been first carried to the west as far as the one force 

 would carry it, and then back towards the east as far as the 

 other would carry it, that is, precisely the same distance ; 

 being ultimately left where it was found at first. 



All laws of causation are liable to be in this manner 

 counteracted, and seemingly frustrated, by coming into con 

 flict with other laws, the separate result of which is opposite 

 to theirs, or more or less inconsistent with it. And hence, 

 with almost every law, many instances in which it really is 

 entirely fulfilled, do not, at first sight, appear to be cases of 

 its operation at all. It is so in the example just adduced: a 

 force, in mechanics, means neither more nor less than a cause 

 of motion, yet the sum of the effects of two causes of motion 

 may be rest. Again, a body solicited by two forces in direc 

 tions making an angle with one another, moves in the diago 

 nal ; and it seems a paradox to say that motion in the diagonal 



