220 INDUCTION. 



We have thus ah-eady come within sight of a conclusion, which the 

 progress of the inquiry will, I think, bring before us with, the clearest 

 evidence : namely, that in the sciences which deal with phenomena in 

 which artificial experiments are impossible (as in the case of astron- 

 omy), or in which they have a very limited range (as in physiology,' 

 m^'ental philosophy, and the social science), indviction from dhect 

 experience is practised at a disadvantage generally equivalent to 

 impracticability : from which it follows that the methods of those 

 sciences, in order to accomplish anything worthy of attainment, must 

 be to a great extent, if not principally, deductive. This is already 

 known to be the case with tlie first of the sciences we have men- 

 tioned, astronomy ; that it is not generally recognized as true of the 

 others, is probably one of the reasons why they are still in their 

 infancy. But any further notice of this topic would at present be 

 premature. 



§ 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 

 connected 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 caiises of a given 

 effect. The fact that light blackens chloride of silver might have been 

 discovered, either by experiments upon light, trying what effect it 

 would produce on various substances, or by observing that portions of 

 the chloride had repeatedly become 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 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 dieorotical discussion, 

 that artificial experimentation is applicable only to the former of these 

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

 produce : but we cannot take an effect, and try what it will be pro- 

 duced by. We can only watch till we see it ]iroduced, 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 under- 

 take 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 contiive, instances of 

 th,e 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 resoui'ce but to comjiare 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 



