286 INDUCTION. 



The difficulty, therefore, in applying the methods already treated of to 

 determine the effects of Permanent Causes, is confined to the cases in 

 which it is impossible for us to get out of the local limits of their influ- 

 ence. The pendulum can be removed from the influence of the mountain, 

 but it can not be removed from the influence of the earth : we can not take 

 away the earth from the pendulum, nor the pendulum from the earth, to 

 ascertain whether it would continue to vibrate if the action which the 

 earth exerts upon it were withdrawn. On what evidence, then, do we 

 ascribe its vibrations to the earth's influence? Not on any sanctioned by 

 the Method of Difference ; for ono of the two instances, the negative in- 

 stance, is wanting. Nor by the Method of Agreement ; for though all 

 pendulums agree in this, that during their oscillations the earth is always 

 present, why may we not as well ascribe the phenomenon to the sun, which 

 is equally a co-existent fact in all the experiments ? It is evident that to 

 establish even so simple a fact of causation as this, there was required 

 some method over and above those which we have yet examined. 



As another example, let us take the phenomenon Heat. Independently 

 of all hypothesis as to the real nature of the agency so called, this fact is 

 certain, that we are unable tD exhaust any body of the whole of its heat. 

 It is equally certain that no one ever perceived heat not emanating from a 

 body. Being unable, then, to separate Body and Heat, we can not effect 

 such a variation of circumstances as the foregoing three methods require ; 

 we can not ascertain, by those methods, what portion of the phenomena 

 exhibited by any body is due to the heat contained in it. If we could ob- 

 serve a body witli its heat, and the same body entirely divested of heat, 

 the Method of Difference would show the effect due to the heat, apart 

 from that due to the body. If we could observe heat under circumstances 

 agreeing in nothing but heat, and therefore not characterized also by the 

 presence of a body, we could ascertain the effects of heat, from an instance 

 of heat with a body and an instance of heat without a body, by the Meth- 

 od of Agreement; or we could determine by the Method of Difference 

 what effect was due to the body, when the remainder which was due to the 

 heat would be given by the Method of Residues, But wo can do none of 

 these things ; and without them the application of any of the three meth- 

 ods to the solution of this problem would be illusory. It would be idle, 

 for instance, to attempt to ascertain the effect of heat by subtracting from 

 the phenomena exhibited by a body all that is due to its other properties; 

 for as we have never been able to observe any bodies without a portion of 

 heat in them, effects due to that heat might form a part of the very re- 

 sults which we were affecting to subtract, in order that the effect of heat 

 might be shown by the residue. 



If, therefore, there were no other methods of experimental investigation 

 than these three, we should be unable to determine the effects due to heat 

 as a cause. But we have still a resource. Though we can not exclude an 

 antecedent altogether, we may be able to produce, or nature may produce 

 for us some modification in it. By a modification is here meant, a change 

 in it not amounting to its total removal. If some modification in the an- 

 tecedent A is always followed by a change in the consequent a, the other 

 consequents b and c remaining the same ; or vick. versa, if every change in 

 a is found to have been preceded by some modification in A, none being 

 observable in any of the other antecedents, we may safely conclude that a 

 is, wholly or in part, an effect traceable to A, or at least in some way con- 

 nected with it through causation. For example, in the case of heat, though 



