THE GEOMETRY OF MOTION 199 



our perceptual experience.^ Rigidity is thus seen to be a 

 conceptual limit, which by concentrating our attention on 

 a special group of perceptions forms a valuable method of 

 classification. 



Although for the description of some types of motion 



it may be useful to replace the wooden chair by a body 



of ideal rigidity in our conceptual map, still the physicist 



tells us that for the purpose of classifying other phases of 



sense-impression, he is bound to consider that the chair is 



not rigid, and that he is perceptually able to measure 



changes in the relative position of its parts. He cannot 



describe the mechanical action between different parts of 



the chair without supposing it elastic, and this elasticity 



involves changes of form in its parts. For example, the 



action between the parts of the chair changes, when it is 



supported on its back instead of its legs, and thus the 



chair changes its form in these two positions. A like 



change of form will take place even if the chair be only 



rotating. Nor does this variation in shape merely result 



from the chair being of wood — it would be equally true 



if the chair were of iron, or any other material. Change 



of form is in many cases perceptually appreciable, and in 



most cases we can determine its conceptual value. Thus, 



so far from the rigid body being a limit which might be 



reached in perception, our whole perceptual experience 



seems to indicate that the conception rigidity corresponds 



to nothing whatever in the real world of phenomena. We 



perceive that most bodies do change their form, and where 



we do not perceive it physics compel us to conceive it. 



1 We speak, for example, of the "distance" from London to Cambridge 

 being fifty-five miles, and this is a practical method of describing the sense- 

 impressions of a journey from one place to the other, and distinguishing it 

 from a journey of fifty-six or fifty-seven miles. But what do we exactly 

 mean ? From Stepney Church to St. Mary's ? If so, from which part of one 

 church to which part of the other ? Or, again, is it from the stone near the 

 gateway of Stepney Church to the last milestone by St. Mary's ? If so, from 

 which side of the one stone to which side of the other ? In the end we find 

 ourselves driven to the conception of a point on either stone — Xio percept iial 

 rpark gets over the difficulty of the where to the where. We are forced to 

 conclude that the idea of distance is a conception reached as a limit to the 

 perceptual, invaluable for classifying our experience but not accurately corre- 

 sponding to a perceptual reality. 



