OF THE LAWS OF MOTION. 



169 



observation of the external world : but there is a constant wish and 

 propensity to express these rules in such terms as shall make them 

 appear self-evident, because identical with the universal and necessary 

 rules of causation. And this propensity is essential to the progress of 

 our knowledge ; and in the success of this effort consists, in a great 

 measure, the advance of the science to its highest point of simplicity 

 and generality. 



22. The nature of the truth which belongs to the laws of motion 

 will perhaps appear still more clearly, if we state, in the following- 

 tabular form, the analysis of each law into the part which is necessary, 

 and the part which is empirical. 



First 

 Law. 



Second 

 Law. 



Third 

 Law. 



Necessary. 



Velocity does not change 

 without a cause. 



The accelerating quantity 

 of a force is measured by the 

 acceleration produced. 



Reaction is equal and op- 

 posite to action. 



Empirical. 



The time for which a body has al- 

 ready been in motion is not a cause of 

 change of velocity. 



The velocity and direction of the mo- 

 tion which a body already possesses are 

 not, either of them, causes which 

 change the acceleration produced. 



The connexion of the parts of a body, 

 or of a system of bodies, and the action 

 to which the body or system is already 

 subject, are not, either of them, causes 

 which change the effects of any ad- 

 ditional action. 



Of course, it will be understood that, when we assert that the con- 

 nexion of the parts of a system does not change the effect of any 

 action upon it, we mean that this connexion does not introduce any 

 new cause of change, but leaves the effect to be determined by the 

 previously established rules of equilibrium and motion. The connexion 

 will modify the application of such rules ; but it introduces no ad- 

 ditional rule: and the same observation applies to all the above stated 

 empirical propositions. 



