THE FOUR EXPERIMENTAL METHODS. 461 



except in some one property. The only mode, there- 

 fore, of prosecuting this inquiry is that afforded by the 

 Method of Agreement ; by which, in fact, through a 

 comparison of all the known substances which had 

 the property of doubly refracting light, it was ascer- 

 tained that they agreed in the single circumstance of 

 being crystalline substances ; and although the con- 

 verse does not hold, although all crystalline substances 

 have not the property of double refraction, it was 

 concluded, with reason, that there is a real connection 

 between these two properties ; that either crystalline 

 structure, or the cause which gives rise to that struc- 

 ture, is one of the conditions of double refraction. 



Out of this employment of the Method of Agree- 

 ment arises a peculiar modification of that method, 

 which is sometimes of great avail in the investigation 

 of nature. In cases similar to the above, in which it 

 is not possible to obtain the precise pair of instances 

 which our second canon requires instances agreeing 

 in every antecedent except A, or in every consequent 

 except a ; we may yet be able, by a double employ- 

 ment of the Method of Agreement, to discover in what 

 the instances which contain A or a, differ from those 

 which do not. 



If we compare various instances in which a occurs, 

 and find that they all have in common the circum- 

 stance A, and (as far as can be observed) no other 

 circumstance, the Method of Agreement, so far, bears 

 testimony to a connexion between A and a. In order 

 to convert this proof of connexion into proof of 

 causation by the direct Method of Difference, we 

 ought to be able in some one of these instances, as 

 for example ABC, to leave out A, and observe 

 whether by doing so, a is prevented. Now supposing 

 (what is often the case) that we are not able to try 



