THE FOUR EXPERIMENTAL METHODS. 463 



a agree in nothing but the 'negation of A. Now if it 

 were possible, which it never is, to have this assurance, 

 we should not need the joint method ; for either of 

 the two sets of instances separately would then be 

 sufficient to prove causation. This indirect method, 

 therefore, can only be viewed as a great extension and 

 improvement of the Method of Agreement, but not as 

 participating in the more cogent nature of the Method 

 of Difference. The following may be stated as its 

 canon : 



THIRD CANON. 



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 circum* 

 stance ; the circumstance in which alone the two sets of 

 instances differ, is the effect, or cause, or a necessary part 

 of the cause, of the phenomenon. 



We shall presently show that the Joint Method 

 of Agreement and Difference constitutes, in another 

 respect not yet adverted to, an improvement upon the 

 common Method of Agreement, namely, in being 

 unaffected by a characteristic imperfection of that 

 method, the nature of which still remains to be 

 pointed out. But as we cannot enter iiito this expo- 

 sition without introducing a new element of com- 

 plexity into this long and intricate discussion, I shall 

 postpone it to the next chapter, and shall at once 

 proceed to the statement of two other methods, which 

 will complete the enumeration of the means which 

 mankind possess for exploring the laws of nature by 

 specific observation and experience. 



