314 INDUCTION. 



not, B must have been absent as well as A, and it would not be true that 

 these instances agree only in not containing A. This, therefore, consti- 

 tutes an immense advantage of the joint method over the simple Method 

 of Agreement. It may seem, indeed, that the advantage does not belong 

 so much to the joint method, as to one of its two }3reraises (if they may be 

 so called), the negative premise. The Method of Agreement, when applied 

 to negative instances, or those in which a phenomenon does not take place, 

 is certainly free from the characteristic imperfection which affects it in the 

 affirmative case. The negative premise, it might therefore be supposed, 

 could be worked as a simple case of the Method of Agreement, without re- 

 quiring an affirmative premise to be joined with it. But though this is 

 true in principle, it is generally altogether impossible to work the Method 

 of Agreement by negative instances without positive ones ; it is so much 

 ra^ore difficult to exhaust the field of negation than that of affirmation. 

 For instance, let the question be M'hat is the cause of the transparency of 

 bodies; with what prospect of success could we set ourselves to inquire 

 directly in what the multifarious substances which are not transparent 

 agree? But we might hope much sooner to seize some point of resem- 

 blance among the comparatively few and definite species of objects which 

 are transparent ; and this being attained, we should quite naturally be put 

 upon examining whether the absence of this one circumstance be not pre- 

 cisely the point in which all opaque substances will be found to resemble. 



The Joint Method of Agreement and Difference, therefore, or as I have 

 otherwise called it, the Indirect Method of Difference (because, like the 

 Method of Difference properly so-called, it proceeds by ascertaining how and 

 in what the cases where the phenomenon is present differ from those in which 

 it is absent) is, after the Direct Method of Difference, the most powerful 

 of the remaining instruments of inductive investigation ; and in the sciences 

 which depend on pure observation, with little or no aid from experiment, 

 this method, so well exemplified in the speculation on the cause of dew, is 

 the primary resource, so far as direct appeals to experience are concerned. 



§ 3. "We have thus far treated Plurality of Causes only as a possible sup- 

 position, which, until removed, renders our inductions uncertain ; and have 

 only considered by what means, where the plurality does not really exist, 

 we may be enabled to disprove it. But we must also consider it as a case 

 actually occurring in nature, and which, as often as it does occur, our 

 methods of induction ought to be capable of ascertaining and establishing. 

 For this, however, there is required no peculiar method. When an effect 

 is really producible by two or more causes, the process for detecting them 

 is in no way different from that by which we discover single causes. They 

 may (first) be discovered as separate sequences, by separate sets of in- 

 stances. One set of observations or experiments shows that the sun is a 

 cause of heat, another that friction is a source of it, another that percus- 

 sion, another that electricity, another that chemical action is such a source. 

 Or (secondly) the plurality may come to light in the course of collating a 

 number of instances, when we attempt to find some circumstance in which 

 they all agree, and fail in doing so. We find it impossible to trace, in all 

 the cases in which the effect is met with, any common circumstance. We 

 find that we can eliminate all the antecedents; that no one of them is 

 present in all the instances, no one of them indispensable to the effect. 

 On closer scrutiny, however, it appears that though no one is always pres- 

 ent, one or other of several always is. If, on further analysis, we can der 



