4-72 INDUCTION. 



of the cause is followed by a change in the effect. And 

 it does usually happen that when a phenomenon A 

 causes a phenomenon a, any variation in the quantity 

 or in the various relations of A, is uniformly followed 

 by a variation in the quantity or relations of a. To 

 take a familiar instance, that of gravitation. The 

 sun causes a certain tendency to motion in the earth ; 

 here we have cause and effect ; but that tendency is 

 towards the sun, and therefore varies in direction as 

 the sun varies in the relation of position ; and more- 

 over the tendency varies in intensity, in a certain 

 numerical ratio to the sun's distance from the earth, 

 that is, according to another relation of the sun. 

 Thus we see that there is not only an invariable con- 

 nexion between the sun and the earth's gravitation, 

 but that two of the relations of the sun, its position 

 with respect to the earth and its distance from the 

 earth, are invariably connected as antecedents with 

 the quantity and direction of the earth's gravitation. 

 The cause of the earth's gravitating at all, is simply 

 the sun ; but the cause of her gravitating with a given 

 intensity and in a given direction, is the existence of 

 the sun in a given direction and at a given distance. 

 It is not strange that a modified cause, which is in 

 truth a different cause, should produce a different 

 effect. But as the cause is only different in its quan- 

 tity, or in some of its relations, it usually happens 

 that the effect also is only changed in its quantity or 

 its relations. 



Although it is for the most part true that a modi- 

 fication of the cause is followed by a modification of 

 the effect, the Method of Concomitant Variations does 

 not, however, pre-suppose this as an axiom. It only 

 requires the converse proposition ; that anything upon 

 whose modifications, modifications of an effect are 



