OF THE LAWS OF MOTION. 151 



adduced in this controversy. This principle I shall state in the form 

 of an axiom, as follows. 



Axiom I. Every change is produced by a cause. 



It will probably be allowed that this axiom expresses a universal 

 and constant conviction of the human mind ; and that in looking at 

 a series of occurrences, whether for theoretical or practical purposes, 

 we inevitably and unconsciously assume the truth of this axiom. If a 

 body at rest moves, or a body in motion stops, or turns to the right 

 or the left, we cannot conceive otherwise than that there is some 

 cause for this change. And so far as we can found our mechanical 

 principles on this axiom, they will rest upon as broad and deep a 

 basis as any truths which can come within the circle of our know- 

 ledge. 



I shall not attempt to analyse this axiom further. Different per- 

 sons may, according to their different views of such subjects, call it a 

 law of our nature that we should think thus, or a part of the con- 

 stitution of the human mind, or a result of our power of seeing the 

 true relations of things. Such variety of opinion or expression would 

 not affect the fundamental and universal character of the conviction 

 which the axiom expresses; and would therefore not interfere with our 

 future reasonings. 



3. There is another axiom connected with this, which is also a 

 governing and universal principle in all our reasoning concerning 

 causes. It may be thus stated. 



Axiom II. Causes are measured by their effects. 



Every effect, that is, every change in external objects, implies a 

 cause, as we have already said : and the existence of the cause is known 

 only by the effects it produces. Hence the intensity or magnitude of 

 the cause cannot be known in any other manner than by these effects: 

 and, therefore, when we have to assign a measure of the cause, we 

 must take it from the effects produced. 



In what manner the effects are to be taken into account, so as 

 to measure the cause for any particular purpose, will have to be 



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