442 INDUCTION. 



an effect, but a cause, of increase of bulk. If we cannot our- 

 selves produce the variations, we must endeavour, though it is 

 an attempt which is seldom successful, to find them produced 

 by nature in some case in which the pre-existing circumstances 

 are perfectly known to us. 



It is scarcely necessary to say, that in order to ascertain 

 the uniform concomitance of variations in the effect with varia- 

 tions in the cause, the same precautions must be used as in 

 any other case of the determination of an invariable sequence. 

 We must endeavour to retain all the other antecedents un- 

 changed, while that particular one is subjected to the requisite 

 series of variations; or in other words, that we may be war- 

 ranted in inferring causation from concomitance of variations, 

 the concomitance itself must be proved by the Method of 

 Difference. 



It might at first appear that the Method of Concomitant 

 Variations assumes a new axiom, or law of causation in 

 general, namely, that every modification of the cause is fol- 

 lowed 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 moreover the tendency varies in 

 intensity, in a certain numerical correspondence 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 

 connexion 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 its gravitating with 

 a giveD intensity and in a given direction, is the existence of 

 the sun in a given direction and at a given distance. It is not 



