J 54 Mr WHEWELL, ON THE NATURE OF THE TRUTH 



of them. We shall have a law of motion corresponding to each 

 of the above three axioms ; the first law will assert that when no force 

 acts, the properties of the motion will be constant; the second law 

 will assert that when a force acts, its quantity is measured by the 

 effect produced ; the third law will assert that, when one body acts 

 upon another, there will be a reaction, equal and opposite to the 

 action. And so far as the laws are announced in this form, they will 

 be of absolute and universal truth, and independent of any particular 

 experiment or observation whatever. 



But though these laws of motion are necessarily and infallibly 

 true, they are, in the form in which we have stated them, entirely 

 useless and inapplicable. It is impossible to deduce from them any 

 definite and positive conclusions, without some additional knowledge or 

 assumption. This will be clear by stating, as we can now do in a 

 very small compass, the proofs of the laws of motion in the form 

 in which they are employed in mechanical reasonings. 



7. First, of the first Law ; — that a body not acted upon by any force 

 will go on in a straight line with an invariable velocity. 



The body will go on in a straight line : for, at any point of its 

 motion, it has a certain direction, which direction will, by Axiom I, 

 continue unchanged, except some cause make it deviate to one side or 

 other of its former position. But any cause which should make the 

 direction deviate towards any part of space would be a force, and the 

 body is not acted upon by any force. Therefore, the direction cannot 

 change, and the body will go on in the same straight line from the 

 first. 



The body will move with an invariable velocity. For the velocity 

 at any point will, by Axiom I, continue unchanged, except some 

 cause make it increase or decrease. And since, by supposition, the 

 body is not acted upon by any force, there can be no such cause 

 depending upon position, that is, upon relations of space; for any 

 cause of change of motion which has a reference to space is force. 



Therefore there can be no cause of change of motion, except 

 there be one depending upon time, such, for instance, as would exist 



