446 INDUCTION. 



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 wounds 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 

 cannot 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 upon our choice from which of the two ends 

 of the sequence we would undertake our inquiries. 

 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 varieties 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 substance had been exposed to 

 the light. If we knew nothing of the Indian arrows 

 but their fatal effect, accident alone could turn our 

 attention to experiments on the urali : in the regular 

 course of investigation, we could only inquire, or try 



