50 DISSERTATION SECOND. fpA.T ik 



without passing through all the intermediate states. Leib- 

 nitz considers himself as the first who made known this 

 law ; but it is fair to remark, that, in as much as motion 

 is concerned, it was distinctly laid down by Galileo, 1 and 

 ascribed by him to Plato. But, though Leibnitz was not 

 the first to discover the law of continuity, he was the first 

 who regarded it as a principle in philosophy, and used it for 

 trying the consistency of theories, or of supposed laws of na- 

 ture, and the agreement of their parts with one another. It 

 was in this way that he detected the error of Descartes's 

 conclusions concerning the collision of bodies, showing, that 

 though one case of collision must necessarily graduate into 

 another, the conclusions of that philosopher did by no means 

 pass from one to another by such gradual transition. In- 

 deed, for the purpose of such detections, the knowledge of 

 this law is extremely useful ; and I believe few have been, 

 much occupied in the investigations either of the pure or 

 mixed mathematics, who have not often been glad to try 

 their own conclusions by the test which it furnishes. 



Leibnitz considered this principle as known a priori, be- 

 cause if any saltus were to take place, that is, if any change 

 were to happen without the intervention of time, the thing 

 changed must be in two different conditions at the same indi- 

 vidual instant, which is obviously impossible. Whether this 

 reasoning be quite satisfactory or not, the conformity of the 

 law to the facts generally observed, cannot but entitle it to 

 great authority in judging of the explanations and theories of 

 natural phenomena. 



It was the usual error, however, of Leibnitz and his follow- 

 ers, to push the metaphysical principles of science into ex- 

 treme cases, where they lead to conclusions to which it was 



1 Opcre di Galileo, Tom. III. p. 150, and Tom. II. p. 32. 

 Edit. Padova, 1744. 



