32 ORIGIN OF PHYSICAL CONCEPTS 



another in the description of spatial conditions. 

 The common element cannot possibly be supplied 

 either by the data of visual sensation which the blind 

 do not possess, or by the data of passive tactual 

 sensation which the vident hardly ever employ. 

 Une etendue commune se retrouverait a la fois dans 

 les donnees de la vue et dans celles du toucher. The 

 common element is furnished by the common laws 

 and forms of our exertional Activity by means of 

 which and in terms of which we all construct our con- 

 ceptions of the dynamic world of our environment. 



It is from our dynamic Activity also that we 

 derive our conception of Force. Force, though it 

 is studied scientifically in the measurement of the 

 great natural forces which operate constantly, 

 is originally known to us in the stress or pressure 

 to which muscular exertion in contact with a 

 material body gives rise. Such a force if it could 

 be correctly measured, would record the rate at 

 which Energy was undergoing transmutation, and 

 it is from such experience of pressure that our idea 

 of Force is originally derived. 



The mass of bodies is usually measured by their 

 weight, i.e. by gravity. Its absolute measurement 

 must be in terms of momentum. The true estimate 

 of the Energy of a body moving under the impulse 



