THE FOUR EXPERIMENTAL METHODS. 473 



invariably consequent, must be the cause (or con- 

 nected with the cause) of that effect ; a proposition, 

 the truth of which is evident ; for if the thing itself 

 had no influence on the effect, neither could the modi- 

 cations of the thing have any influence. If the stars 

 have no power over the fortunes of men, it is implied 

 in the very terms, that the conjunctions or oppositions 

 of different stars can have no such power. 



Although the most striking applications Of the 

 Method of Concomitant Variations take place in the 

 cases in which the Method of Difference, strictly so 

 called, is impossible, its use is not confined to those 

 cases ; it may often usefully follow after the Method 

 of Difference, to give additional precision to a solution 

 which that has found. When by the Method of Differ- 

 ence it has first been ascertained that a certain object 

 produces a certain effect, the Method of Concomitant 

 Variations may be usefully called in to determine 

 according to what law the quantity or the different 

 relations of the effect follow those of the cause. 



7. The case in which this method admits of the 

 most extensive employment, is that in which the 

 variations of the cause are variations of quantity. Of 

 such variations we may in general affirm with safety, 

 that they will be attended not only with variations, 

 but with similar variations, of the effect : the proposi- 

 tion, that more of the cause is followed by more of 

 the effect, being a corollary from the principle of the 

 Composition of Causes, which, as we have seen, is the 

 general rule of causation ; cases of the opposite 

 description, in which causes change their properties 

 on being conjoined with one another, being, on the 

 contrary, special and exceptional. Suppose, then, 

 that when A changes in quantity, a also changes in 



