276 INDUCTION. 



deductive. This is already known to be the case with the first of the sci- 

 ences we have mentioned, astronomy ; that it is not generally recognized 

 as true of the others, is probably one of the reasons why they are not in a 

 more advanced state. 



§ 4. If what is called pure observation is at so great a disadvantage, 

 compared with artificial experimentation, in one department of the direct 

 exploration of phenomena, there is another branch in which the advantage 

 is all on the side of the former. 



Inductive inquiry having for its object to ascertain what causes are con- 

 nected with what effects, we may begin this search at either end of the road 

 which leads from the one point to the other: we may either inquire into 

 the effects of a given cause or into the causes of a given effect. The fact 

 that light blackens chloride of silver might have been discovered either by 

 experiments on light, trying what effect it would produce on various sub- 

 stances, or by observing that portions of the chloride had repeatedly be- 

 come black, and inquiring into the circumstances. The effect of the urali 

 poison might have become known either by administering it to animals, 

 or by examining how it happened that the Avouuds which the Indians of 

 Guiana inflict with their arrows prove so uniformly mortal. Now it is 

 manifest from the mere statement of the examples, without any theoretical 

 discussion, that artificial experimentation is applicable only to the former of 

 these modes of investigation. We can take a cause, and try what it will 

 produce ; but we can not take an effect, and try what it will be produced 

 by. We can only watch till we see it produced, or are enabled to produce 

 it by accident. 



This would be of little importance, if it always depended on our choice 

 from which of the two ends of the sequence we would undertake our in- 

 quiries. But we have seldom any option. As we can only travel from 

 the known to the unknown, we are obliged to commence at whichever end 

 we are best acquainted with. If the agent is more familiar to us than its 

 effects, we watch for, or contrive, instances of the agent, under such vari- 

 eties of circumstances as are open to us, and observe the result. If, on the 

 contrary, the conditions on which a phenomenon depends are obscure, but 

 the phenomenon itself familiar, we must commence our inquiry from the 

 effect. If we are struck with the fact that chloride of silver has been 

 blackened, and have no suspicion of the cause, we have no resource but to 

 compare instances in which the fact has chanced to occur, until by that 

 comparison we discover that in all those instances the substances had been 

 exposed to light. If we knew nothing of the Indian arrows but their fa- 

 tal effect, accident alone could turn our attention to experiments on the 

 urali ; in the regular course of investigation, we could only inquire, or try 

 to observe, what had been done to the arrows in particular 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 can not, 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 ques- 

 tion are not known to us. We have, therefore, no expedient but to study it 

 where it offers itself spontaneously. If nature happens to pi'esent us with 

 instances sufficiently varied in their circumstances, and if we ai'e able to dis- 



