153 mk whewell, on the nature of the truth 



further considered ; but the axiom, as now stated, is absolutely and 

 universally true, and is acted upon in all parts of our knowledge in 

 which causes are measured. 



4. But something further is requisite. We not only consider that 

 all changes of motion in a body have a cause, but that this cause may 

 reside in other bodies. Bodies are conceived to act upon one another, 

 and thus to influence each other's motions, as when one billiard ball 

 strikes another. But when this happens, it is also supposed that the 

 body struck influences the motion of the striking body. This is inclu- 

 ded in our notion of body or matter. If one ball could strike and 

 affect the motions of any number of others without having its own 

 motion in any degree affected, the struck balls would be considered, 

 not as bodies, but as mere shapes or appearances. Some reciprocal in- 

 fluence, some resistance, in short some reaction, is necessarily involved 

 in our conception of action among bodies. All mechanical action upon 

 matter implies a corresponding reaction; and we might describe matter 

 as that which resists or reacts when acted on by force. Not only 

 must there be a reaction in such cases, but this reaction is defined 

 and determined by the action which produces it, and is of the same 

 kind as the action itself The action which one body exerts upon 

 another is a blow, or a pressure; but it cannot press or strike with- 

 out receiving a pressure or a blow in return. And the reciprocal 

 pressure or blow depends upon the direct, and is determined altogether 

 and solely by that. But this action being mutual, and of the same 

 kind on each body, the effect on each body will be determined by the 

 effect on the other, according to the same rule ; each effect in turn 

 being considered as action and the other as reaction. But this cannot be 

 otherwise than by the equality and opposite direction of the action and 

 reaction. And since this reasoning applies in all cases in which bodies 

 influence each others motions, we have the following axiom which is 

 universally true, and is a fundamental principle with regard to all me- 

 chanical relations. 



Axiom III. Action is always accompanied by an equal and opposite 



Reaction. 



