288 INDUCTION. 



The last clause is subjoined, because it by no means follows when two 

 phenomena accompany each other in their variations, that the one is cause 

 and the other effect. The same thing may, and indeed must happen, sup- 

 posing them to be two different effects of a common cause : and by this 

 method alone it would never be possible to ascertain which of the suppo- 

 sitions is the true one. The only way to solve the doubt would be that 

 which we have so often adverted to, viz., by endeavoring to ascertain wheth- 

 er we can produce the one set of variations by means of the other. In the 

 case of heat, for example, by increasing the temperature of a body we in- 

 crease its bulk, but by increasing its bulk we do not increase its temper- 

 ature; on the contrary (as in the rarefaction of air under the receiver 

 of an aii'-pump), we generally diminish it: therefore heat is not an effect, 

 but a cause, of increase of bulk. If we can not ourselves produce the va- 

 riations, we must endeavor, though it is an attempt which is seldom suc- 

 cessful, 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 con- 

 comitance of variations in the effect with variations in the cause, the same 

 precautions must be used as in any other case of the determination of an 

 invariable sequence. We must endeavor to retain all the other anteced- 

 ents unchanged, while that particular one is subjected to the requisite se- 

 ries of variations ; or, in other words, that we may be warranted in infer- 

 ring 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 as- 

 sumes a new axiom, or law of causation in general, namely, that every mod- 

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

 usually happen that when a phenomenon A causes a phenomenon «, any 

 variation in the quantity or in the various relations of A, is uniformly fol- 

 lowed 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 toward 

 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, ac- 

 cording to another relation of the sun. Thus we see that there is not 

 only an invariable connection between the sun and the earth's gravitation, 

 but that two of the relations of the sun, its position wdth respect to the 

 earth and its distance from the earth, are invariably connected as anteced- 

 ents 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 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. 



Although it is for the most part true that a modification of the cause is 

 followed by a modification of the effect, the Method of Concomitant Varia- 

 tions does not, however, presuppose this as an axiom. It only requires 

 the converse proposition : that any thing on whose modifications, modifi- 

 cations of an effect are 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 modifications of the thing have any influence. If the stars have no 



