THE GEOMETRY OF MOTION 205 



changes of angle. But in the cases of both slide and stretch 

 we are thrown back on geometrical notions, when we 

 come to consider their measurement ; in both cases we 

 replace the perceptual body by a conceptual body built 

 up of points, lines, and angles. Thus the whole theory of 

 strain deals with a conceptual means of distinguishing and 

 describing perceptions, and not with something actually 

 inherent in those perceptions themselves. 



S 6. — Factors of Coticeptiial Motion 



We started with a man ascending a staircase, and we 

 have seen by our analysis that the conceptual description 

 of his motion requires us to discuss : {a) The Motion of a 

 Point, iU) the Motion of a Rigid Body about a Fixed 

 Point, ic) the Relative Motion of the Parts of a Body or 

 its Strain. These are the three great divisions of Kine- 

 matics, or the Geometry of Motion. But in the case of 

 all these divisions we find that we are thrown back on the 

 ideal conceptions of geometry ; we measure distances 

 between points and angles between lines, which are not 

 true limits to our perceptual experience. Thus our ideas 

 of motion appear as ideal modes, in terms of which we 

 describe and classify the sequences of our sense-impres- 

 sions : they are purely symbols by aid of which we resume 

 and index the various and continual changes undergone 

 by the picture our perceptive faculty presents to us. The 

 more fully and clearly the reader grasps this fact, the more 

 readily will he admit that science is a conceptual description 

 and classification of our perceptions, a theory of symbols 

 which economises thought. It is not an explanation of 

 anything. It is not 2. plan which lies in phenomena them- 

 selves. Science may be described as a classified index 

 to the successive pages of sense-impression which enables 

 us readily to find what we want, but it in nowise accounts 

 for the peculiar contents of that strange book of life.^ 



tributed by Ihe present writer to chapter iii. of Clifford's Common Sense of 

 the Exact Sciences. 



1 The extremely complex results which flow from the simple basis of the 

 planetary theory have often been taken as an evidence of "design" in the 



