iict. ii.] DISSERTATION SECOND. 40 



which determine two different things, whether magnitudes or 

 events, are in two cases precisely the same, it cannot be 

 doubted that these events or magnitudes are in all respects 

 identical. 



However sound this principle may be in itself, the use 

 which Leibnitz sometimes made of it has tended to bring it 

 into discredit. He argued, for example, that of the particles 

 of matter no two can possess exactly the same properties, or 

 can perfectly resemble one another, otherwise the Supreme 

 Being could have no reason for employing one of them in a 

 particular position more than another, so that both must ne- 

 cessarily be rejected. To argue thus, is to suppose that we 

 completely understand the manner in which motives act on 

 the mind of the Divinity, 1 a postulate that seems but ill suit- 

 ed to the limited sphere of the human understanding. But, 

 if Leibnitz has misapplied his own principle and extended 

 its authority too far, this affords no ground for rejecting it 

 when we are studying the ordinary course of nature, and ar- 

 guing about the subjects of experiment and observation. In 

 fact, therefore, the sciences which aspire to place their 

 foundation on the solid basis of necessary truth, are much 

 indebted to Leibnitz for the introduction of this principle into 

 philosophy. 



Another principle of great use in investigating the laws 

 of motion, and of change in general, was brought into 

 view by the same author, — the law of Continuity. — accord- 

 ing to which, nothing passes from one state to another 



1 The argument of Leibnitz seems evidently inconclusive. 

 For, though there were two similar and equal atoms, yet as they 

 could not co-exist in the same space, they would not, so far as 

 position is concerned, bear the same relation to the particles 

 that surrounded them ; there might exist, therefore, consider- 

 ing them as part of the materials to be employed in the con- 

 struction of the universe, very good reasons for assigning diffe- 

 rent situations to each. 



