vi CONCERNING NECESSARY TRUTHS 149 



cover to be common amongst them. For as like effects 

 imply like causes, we must always ascribe the causation to 

 the circumstance wherein we discover the resemblance." 

 (I. p. 229.) 



Next, the foundation of the method of difference 

 is stated: 



" The difference in the effects of two resembling objects 

 must proceed from that particular in which they differ. 

 For, as like causes always produce like effects, when in any 

 instance we find our expectation to be disappointed, we 

 must conclude that this irregularity proceeds from some 

 difference in the causes." (I. p. 230.) 



In the succeeding paragraph the method of con- 

 comitant variations is foreshadowed. 



" When any object increases or diminishes with the 

 increase or diminution of the cause, 'tis to be regarded as a 

 compounded effect, derived from the union of the several 

 different effects which arise from the several different parts 

 of the cause. The absence or presence of one part of the 

 cause is here supposed to be always attended with the ab- 

 sence or presence of a proportionable part of the effect. 

 This constant conjunction sufficiently proves that the one 

 part is the cause of the other. We must, however, beware 

 not to draw such a conclusion from a few experiments." 

 (I. p. 230.) 



Lastly, the following rule, though awkwardly 

 stated, contains a suggestion of the method of 

 residues : 



"... an object which exists for any time in its full 

 perfection without any effect, is not the sole cause of that 

 effect, but requires to be assisted by some other principle, 

 which may forward its influence and operation. For as 



