458 INDUCTION. 



preexisting state of circumstances a change perfectly 

 definite. We choose a previous state of things with 

 which we are well acquainted, so that no unforeseen 

 alteration in that state is likely to pass unobserved ; 

 and into this we introduce, as rapidly as possible, the 

 phenomenon which we wish to study ; so that we in 

 general are entitled to feel complete assurance, that 

 the pre-existing state, and the state which we have 

 produced, differ in nothing except in the presence or 

 absence of that phenomenon. If a bird is taken from 

 a cage, and instantly plunged into carbonic acid gas, 

 the experimentalist may be fully assured (at all events 

 after one or two repetitions) that no circumstance 

 capable of causing suffocation had supervened in the 

 interim, except the change from immersion in the 

 atmosphere to immersion in carbonic acid gas. There 

 is one doubt, indeed, which may remain in some cases 

 of this description ; the effect may have been pro- 

 duced not by the change, but by the means we 

 employed to produce the change. The possibility, 

 however, of this last supposition generally admits of 

 being conclusively tested by other experiments. It 

 thus appears that in the study of the various kinds of 

 phenomena which we can, by our voluntary agency, 

 modify or control, we can in general satisfy the requi- 

 sitions of the Method of Difference ; but that by the 

 spontaneous operations of nature those requisitions 

 are seldom fulfilled. 



The reverse of this is the case with the Method 

 of Agreement. We do not here require instances of 

 so special and determinate a kind. Any instances 

 whatever, in which nature presents us with a pheno- 

 menon, may be examined for the purposes of this 

 method ; and if all such instances agree in any- 

 thing, a conclusion of considerable value is already 



