r8o THE SCIENCE OF LOGIC 



hand, and that we seek to make good its limitations by experi 

 mental observation of a number of instances especially of nega 

 tive instances varying among themselves in accordance with the 

 method of agreement. 



The DOUBLE METHOD OF AGREEMENT is stated thus by Dr. 

 Mellone 1 : &quot;WHATEVER is PRESENT IN NUMEROUS OBSERVED 



INSTANCES OF THE PRESENCE OF A PHENOMENON, AND ABSENT 

 IN NUMEROUS OBSERVED INSTANCES OF ITS ABSENCE, IS PROB 

 ABLY CONNECTED CAUSALLY WITH THE PHENOMENON&quot;. 



The two sets of instances must, of course, be drawn from the 

 same field of investigation ; they must be in pan materia ; they 

 will, therefore, have a great deal in common (the more the better) ; 

 each negative instance will be so chosen as to resemble as much 

 as possible some positive instance ; if any such pair could be pro 

 cured with everything in common (save the supposed cause), we 

 should have the requisite data for the method of difference, and we 

 might not have recourse to the present method at all : it is pre 

 cisely because we cannot by simple observation procure two such 

 instances that we must, as the next best course, apply the present 

 method ; and in applying it we select positive instances which 

 vary as much as possible among themselves, and likewise nega 

 tive instances which, like the positive ones, vary as much as 

 possible among themselves. Of all these points we have a clear 

 and easy illustration in the following example : 2 



&quot; A fever has broken out in a town ; what is the cause ? 

 The patients vary in age, general health, circumstances, etc., but 

 they are all supplied with milk from the same dairy. You sus 

 pect (by the method of agreement) that some taint in the milk has 

 caused the fever. Suppose the dairyman pleads the plurality 

 of causes ; viz. the possibility that some of the patients have got 

 the fever by direct infection, others from bad drains, others from 

 poor living, none from the milk. How would you answer him? 

 You would see whether those who did not drink the milk from 

 his dairy were also free from the fever. If you could say, here 

 are a number of people living under much the same circumstances 

 as the fever patients, some exposed to direct infection, some to 

 bad drains, some to semi-starvation, but none of them drink the 

 milk from your dairy and none of them have fever, then the case 

 against the milk would be much strengthened ; because you could 



l op. cit., p. 306. 



2 Taken from the Palaestra Logica, p. in, 346. Cf. infra, 245. 



