THE FOUR EXPERIMENTAL METHODS. 281 



-§ 3. The two methods which we have now stated have many features 

 of resemblance, but there are also many distinctions between them. Both 

 are methods of elimination. This term (employed in the theory of equa- 

 tions to denote the process by which one after another of the elements of 

 a question is excluded, and the solution made to depend on the relation 

 between the remaining elements only) is well suited to express the opera- 

 tion, analogous to this, which has been understood since the time of Bacon 

 to be the foundation of experimental inquiry : namely, the successive ex- 

 clusion of the various circumstances which are found to accompany a phe- 

 nomenon in a given instance, in order to ascertain what are those among 

 them which can be absent consistently with the existence of the phenome- 

 non. The Method of Agreement stands on the ground that whatever can 

 be eliminated, is not connected with the phenomenon by any law. The 

 Method of Difference has for its foundation, that whatever can not be 

 eliminated, is connected with the phenomenon by a law. 



Of these methods, that of Difference is more particularly a method of 

 artificial experiment ; while that of Agreement is more especially the re- 

 source employed where experimentation is impossible. A few reflections 

 will prove the fact, and point out the reason of it. 



It is inherent in the peculiar character of the Method of Difference, that 

 the nature of the combinations which it requires is much more strictly de- 

 fined than in the Method of Agreement. The two instances which are to 

 be compared with one another must be exactly similar, in all circumstances 

 except the one which we are attempting to investigate : they must be in 

 the relation of A B C and B C, or oi ab c and b c. 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 commonest experience, that most of the co-existent 

 phenomena of the universe may be either present or absent without affect- 

 ing the given phenomenon ; or, if present, are present indifferently when 

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

 the identity Avhich is required between the two instances, ABC and B C, 

 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 spontane- 

 ous operations of nature there is generally such complication and such ob- 

 scurity, they are mostly either on so overwhelmingly large or on so inac- 

 cessibly 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 Dif- 

 fei'ence, 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 circum- 

 stances 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. Comto 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 pos- 



