158 Mr WHEWELL, ON THE NATURE OF THE TRUTH 



portion of the process of proof consisted in shewing that in those cases 

 in which the accelerative effect of a force appeared to be changed by 

 the circumstances of the motion of the body acted on, the change was, 

 in fact, due to other external forces ; so that all evidence of a cause 

 of change residing in those circumstances was entirely negatived; and 

 thus the law, that the accelerative effect of the same force is the 

 same, appeared to be absolutely and rigorously true. 



9. When the motions of bodies are not affected merely by forces 

 like gravity, which are only perceived by their effects, but are acted 

 upon by other bodies, the case requires other considerations. 



It is in such cases that we originally form the conception of force; 

 we ourselves pull and push, thrust and throw bodies, with a view, it 

 may be, either to put them in motion, or to prevent their moving, 

 or to alter their figure. Such operations, and the terms by which 

 they are described, are all included in the term force, and in other 

 terms of cognate import. And in using this term, we necessarily 

 assume and imply the co-existence of these various effects of force 

 which we have observed universally to accompany each other. Thus 

 the same kind of force which is the cause of motion, may also be 

 the cause of a body having a form different from its natural form ; 

 when we draw a bow, the same kind of pull is needed to move the 

 string, and to hold it steady when the bow is bent. And a weight 

 might be hung to the string, so as to produce either the one or 

 the other of these effects. By an infinite multiplicity of experiments 

 of this kind, we become imbued with the conviction that the same 

 pressure may be the cause of tension and of motion. Also as the 

 cause can be known by its effects only, each of these effects may be 

 taken as its measure ; and therefore, so long as one of them is the 

 same, since the cause is the same, the other must be the same also. 

 That is, so long as the pressure or force which shews itself in 

 tension is the same, the motion which it would produce must, under 

 the same circumstances, be the same also. This general fact is not 

 a result of any particular observations, but of the general observation 

 or suggestion arising unavoidably from universal experience, that both 



