166 Mb WHEWELL, ON THE NATURE OF THE TRUTH 



a cause, — that change of action is proportional to the force which pro- 

 duces it, — and that action and reaction are equal and opposite. The 

 truth of these assertions is involved in those notions of causation and 

 matter, which the very attempt to know any thing concerning the rela- 

 tions of matter and motion presupposes. But, according to the facts 

 which we might find, in such imaginary cases as I have spoken of, 

 we should settle in a different way — what is a cause of change of ve- 

 locity, — what is the measure of the force which changes motion, — and 

 what is the measure of action between bodies. The law is necessary, 

 if there is to be a law ; the meaning of its terms is decided by what 

 we find, and is therefore regulated by our special experience. 



19. It may further illustrate this matter to point out that this 

 view is confirmed by the history of mathematics. The laws of motion 

 were assented to as soon as propounded; but were yet each in its turn 

 the subject of strenuous controversy. The terms of the law, the form, 

 which is necessarily true, were recognised and undisputed ; but the 

 meaning of the terms, the substance of the law, was loudly contested; 

 and though men often tried to decide the disputed points by pure 

 reasoning, it was easily seen that this could not suffice ; and that since 

 it was a case where experience could decide, experience must be the 

 proper test: since the matter came within her jurisdiction, her authority 

 was single and supreme. 



Thus with regard to the first law of motion, Aristotle allowed that 

 natural motions continue unchanged, though he asserted the motions 

 of terrestrial bodies to be constrained motions, and therefore, liable to 

 diminution. Whether this was the cause of their diminution was a 

 question of fact, which was, by examination of facts, decided against 

 Aristotle. In like manner, in the first case of the second law of 

 motion which came under consideration, both Galileo and his oppo- 

 nent agree that falling bodies are uniformly accelerated ; that is, that 

 the force of gravity accelerates a body uniformly whatever be the 

 velocity it has already ; but the question arises, what is uniform acce- 

 leration ? It so happened in this case, that the first conjecture of Ga- 

 lileo, afterwards defended by Casraeus, (that the velocity was propor- 



