THE FOUR EXPERIMENTAL METHODS. 467 



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 were withdrawn.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 

 one of the two instances, the negative instance, is 

 wanting. Nor by the Method of Agreement ; for 

 although 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 coexistent fact in all the experi- 

 ments ? 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 to 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 cannot 

 effect such a variation of circumstances as the fore- 

 going three methods require ; we cannot ascertain, by 

 those methods, what portion of the phenomena 

 exhibited by any body are due to the heat contained 

 in it. If we could observe a body with 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 Method of Agreement ; 

 or, if we pleased, we could determine by the Method 



2 H 2 



