DISCOVERY OF THE LAWS OF MOTION. 25 



motion to be removed; it is then manifest, from 

 what has been said more at large in another place, 

 that the body's motion will be uniform and per- 

 petual upon the plane, if the plane be indefinitely 

 extended." His pupil, Borelli, in 1667 (in the 

 treatise De Vi Percussionis), states the proposition 

 generally, that " Velocity is, by its nature, uniform 

 and perpetual;" and this opinion appears to have 

 been, at that time, generally diffused, as we find 

 evidence in Wallis and others. It is commonly said 

 that Descartes was the first to state this generally. 

 His Principia were published in 1644; but his 

 proofs of this First Law of Motion are rather of 

 a theological than of a mechanical kind. His rea- 

 son for this Law is 3 , "the immutability and sim- 

 plicity of the operation by which God preserves 

 motion in matter. For he only preserves it pre- 

 cisely as it is in that moment in which he preserves 

 it, taking no account of that which may have been 

 previously." Reasoning of this abstract and a 

 priwi kind, though it may be urged in favour of 

 true opinions after they have been inductively esta- 

 blished, is almost equally capable of being called 

 in on the side of errour, as we have seen in the 

 case of Aristotle's philosophy. We ought not, how- 

 ever, to forget that the reference to these abstract 

 and a priori principles is an indication of the 

 absolute universality and necessity which we look 

 for in complete sciences, and a result of those 

 3 Princip. p. 34. 



