66 INDUCTION. 



produce a tendency to error peculiarly in one direc- 

 tion, we are warranted by experience in assuming that 

 the errors on one side will, in a certain number of 

 experiments, about balance the errors on the contrary 

 side. We have, therefore, to repeat the experiment, 

 until any change which is produced in the average of 

 the whole by further repetition, falls within limits of 

 error consistent with the degree of accuracy required 

 by the purpose we have in view. 



^ 4. In the supposition hitherto made, the effect of 

 the constant cause A has been assumed to form so great 

 and conspicuous a part of the general result, that its 

 existence never could be a matter of uncertainty, and 

 the object of the eliminating process was only to ascer- 

 tain how much is attributable to that cause ; what is 

 its exact law. Cases, however, occur in which the 

 effect of a constant cause is so small, compared with 

 that of some of the changeable causes with which it is 

 liable to be casually conjoined, that of itself it escapes 

 notice, and the very existence of any effect arising 

 from a constant cause is first learnt, by the process 

 which in general serves only for ascertaining the quan- 

 tity of that effect. This case of induction may be 

 characterized as follows. A given effect is known to 

 be chiefly, and not known not to be wholly, deter- 

 mined by changeable causes. If it be wholly so pro- 

 duced, then if the aggregate be taken of a sufficient 

 number of instances, the effects of these different 

 causes will cancel one another. If, therefore, we do 

 not find this to be the case, but, on the contrary, after 

 such a number of trials has been made that no further 

 increase alters the average result, we find that average 

 to be, not zero, but some other quantity, around which, 

 though small in comparison with the total effect, the 



