THE FOUR EXPERIMENTAL METHODS. 457 



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 learn 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 phenomenon ; 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 such circumstances 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 spontaneous operations of 

 nature there is generally such complication and such 

 obscurity, they are mostly either on so overwhelm- 

 ingly 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. When, 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, 

 provided the process does not last a long time. A 

 certain state of surrounding circumstances existed 

 before we commenced 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 



