226 INDUCTION. 



that the nature of the combinations which it requires ns much more 

 strictly defined than in the Method of Agi-eement, The two instances 

 which are to be compared with one another must be exactly similar, 

 in all circumstances except the one which we are attemj)ting to inves- 

 tigate : they must be in the relation of ABC and BC, or of a Z»c and 

 be. It is true that this similarity of circumstances needs not extend 

 to such as are already known to be immaterial to the result. And in 

 the case of most phenomena we leani at once, from the most ordinary 

 experience, that most of the coexistent phenomena of the universe 

 may be either present or absent without affecting the given phenome- 

 non ; or, if present, are present indifferently when the phenomenon' 

 does not happen, and when it does. Still, even limiting the identity 

 which is required between the two instances, ABC and BC, to snch 

 circumstanceis as are not already known to be indifferent ; it is very 

 seldom that nature affords two instances, of which we can be assured 

 that they stand in this precise relation to one another. ' In the spon- 

 taneous operations of nature there is generally such complication and 

 such obscurity, they are mostly either on so overwhelmingly large or 

 on so inaccessibly minute a scale, we are' so ignorant of a great part 

 of the facts which really take place, and even those of which we are 

 not ignorant are so multitudinous, and therefore so seldom exactly 

 alike in any two cases, that a spontaneous experiment, of the kind 

 required by the Method of Difference, is commonly not to be found. 

 AVlaen, on the contrary, we obtain a phenomenon by an artificial 

 experiment, a pair of instances such as the method requires is obtained 

 almost as a matter of course, j^rovided the process does not last a long 

 time. A certain state of sun-ounding circumstances existed before we 

 commenced the experiment : this is BC. We then hitroduce A ; say, 

 for instance, by merely bringing an object from another part of Hie 

 room, before there has been time for any change in the other ele- 

 ments. It is, in short (as M. Comte obsei-\-es), the very nature of an 

 experiment, to introduce into the 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 prodviced, 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 ail events after one or two 

 repetitions) that no circumstance capable of causing suffocation had 

 supei-vened 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 produced 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 vohmtary agency, modify or 

 control, we can in general satisfy the requisitions of the Method of 

 Difference ; but that by the spontaneous operations of nature those 

 requisitions are seldom fulfilled. 



