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CHAPTER XIX. 



OF THE EXTENSION OF DERIVATIVE LAWS TO 

 ADJACENT CASES. 



1. WE have had frequent occasion to notice the 

 inferior generality of derivative laws, compared with 

 the ultimate laws from which they are derived. This 

 inferiority, which affects not only the extent of the 

 propositions themselves, but their degree of certainty 

 within that extent, is most conspicuous in the unifor- 

 mities of coexistence and sequence obtaining between 

 effects which depend ultimately upon different pri- 

 meval causes. Such uniformities will only obtain 

 where there exists the same collocation of those pri- 

 meval causes. If the collocation varies, though the 

 laws themselves remain the same, a totally different 

 set of derivative uniformities may, and generally will, 

 be the result. 



Even where the derivative uniformity is between 

 different effects of the same cause, it will by no means 

 obtain as universally as the law of the cause itself. If 

 a and b accompany or succeed one another as effects 

 of the cause A, it by no means follows that A is the 

 only cause which can produce them, or that if there 

 be another cause, as B, capable of producing a, it 

 must produce b likewise. The conjunction, therefore, 

 of a and 6, perhaps does not hold universally, but 

 only in the instances in which a arises from A. When 

 it is produced by a cause other than A, a and b may 

 be dissevered. Day (for example) is always in our 

 experience followed by night ; but day is not the cause 

 of night ; both are successive effects of a common 



