OBSERVATION AND EXPERIMENT. 447 



to observe, what had been done to the arrows in par- 

 ticular instances. 



Wherever, having nothing to guide us to the 

 cause, we are obliged to set out from the effect, and 

 to apply the rule of varying the circumstances to the 

 consequents, not the antecedents, we are necessarily 

 destitute of the resource of artificial experimentation. 

 We cannot, at our choice, obtain consequents, as we 

 can antecedents, under any set of circumstances com- 

 patible with their nature. There are no means of 

 producing effects but through their causes, and by the 

 supposition the causes of the effect in question are not 

 known to us. We have therefore no expedient but 

 to study it where it offers itself spontaneously. If 

 nature happens to present us with instances suffi- 

 ciently varied in their circumstances, and if we are 

 able to discover either among the proximate ante- 

 cedents, or among some other order of antecedents, 

 something which is always found when the effect is 

 found, however various the circumstances, and never 

 found when it is not ; we may discover, by mere 

 observation without experiment, a real uniformity in 

 nature. 



But although this is certainly the most favourable 

 case for sciences of pure observation, as contrasted 

 with those in which artificial experiments are possible, 

 there is in reality no case which more strikingly illus- 

 trates the inherent imperfection of direct induction 

 when not founded upon experimentation. Suppose 

 that, by a comparison of cases of the effect, we have 

 found an antecedent which appears to be, and perhaps 

 is, invariably connected with it: we have not yet 

 proved that antecedent to be the cause, until we have 

 reversed the process, and produced the effect by 

 means of that antecedent. If we can produce the 



