METHOD OF DISCO V BRING CA USAL LAWS 1 8 1 



point to instances in which the other possible causes of the fever 

 had not produced it.&quot; 



&quot; You would not use this method if you could use Difference. 

 But you can neither, if any one drinks the milk and gets the 

 fever, be sure that the milk is the only new antecedent, nor find 

 two people exactly alike in everything likely to produce fever ex 

 cept that one has drunk the milk and the other has not.&quot; 



When we attempt to trace causal connexions between the 

 phenomena which form the subject-matter of the social, politi 

 cal and economic sciences, we shall rarely find it feasible to apply 

 the method of difference simply ; we are obliged, therefore, to 

 have recourse to the double method of agreement, or to &quot; con 

 comitant variations &quot; (243), or &quot; residues &quot; (244). 



Mill s formulation of this double method, which he called the indirect 

 method of difference, or, also, the joint method of agreement and difference, is 

 as follows : 



&quot; If two or more instances in which the phenomenon occurs have only one 

 circumstance in common, while two or more instances in which it does not 

 occur have nothing in common save the absence of that circumstance ; the 

 circumstance in which alone the two sets of instances differ, is the effect, or 

 the cause, or an indispensable part of the cause, of the phenomenon&quot; 



This formula is vague, if not even misleading. Two instances in each 

 set would be rarely, if ever, sufficient. Very rarely can the positive in 

 stances have &quot; only one circumstance in common,&quot; or the negative instances 

 &quot; nothing in common save the absence of that circumstance &quot;. 



Among his examples of this method is the following : &quot; It appears that 

 the instances in which much dew is deposited, which are very various, agree 

 in this, and, so far as we are able to observe, in this only, that they either 

 radiate heat rapidly or conduct it slowly : qualities between which there is no 

 other circumstance of agreement, than that by virtue of either, the body tends 

 to lose heat more rapidly from its surface than it can be restored from within. 

 The instances, on the contrary, in which no dew, or but a small quantity of it, 

 is formed, and which are also extremely various, agree (as far as we can observe) 

 in nothing except in not having this same property. 1 We seem therefore to 

 have detected the characteristic difference between the substances on which 

 dew is produced and those on which it is not produced.&quot; 2 



Let us now see how the combination of difference and agree 

 ment is applied to data which we can control by experiment, in 

 accordance with what we have called the JOINT METHOD OF 



1 i.e. in nothing that is considered likely to influence the presence or absence of 

 dew : for, surely, being in pari materia, concerned with the same group of phe 

 nomena, the instances must have several other positive (though presumably indif 

 ferent) circumstances in common, and must also agree in the absence of several other 

 presumably irrelevant circumstances. 



8 Logic, III., ix., 3. 



