THE FOUR EXPERIMENTAL METHODS. 431 



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 spontaneous 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 ex- 

 actly alike in any two cases, that a spontaneous experiment, of 

 the kind required by the Method of Difference, is commonly 

 not to be found. When, on the contrary, we obtain a pheno- 

 menon by an artificial experiment, a pair of instances such as 

 the method requires is obtained almost as a matter of course, 

 provided the process does not last a long time. A certain 

 state of surrounding circumstances existed before we com- 

 menced the experiment ; this is B C. We then introduce A ; 

 say, for instance, by merely bringing an object from another 

 part of the room, before there has been time for any change 

 in the other elements. It is, in short (as M. Comte observes), 

 the very nature of an experiment, to introduce into the pre- 

 existing 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 in general we are entitled to feel complete assurance 

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

 duced, differ in nothing except 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 descrip- 

 tion ; the effect may have been produced not by the change, 

 but by the means employed to produce the change. The pos- 

 sibility, however, of this last supposition generally admits of 



