SENSE-DATA AND PHYSICS 161 



similarly a perspective is a member of the multiplicative 

 class of the things which appear in it. 



The arrangement of perspectives in a space is effected 

 by means of the differences between the appearances of a 

 given thing in the various perspectives. Suppose, say, 

 that a certain penny appears in a number of different 

 perspectives ; in some it looks larger and in some smaller, 

 in some it looks circular, in others it presents the appear- 

 ance of an ellipse of varying eccentricity. We may collect 

 together all those perspectives in which the appearance oi 

 the penny is circular. These we will place on one straight 

 line, ordering them in a series by the variations in the 

 apparent size of the penny. Those perspectives in which 

 the penny appears as a straight line of a certain thickness 

 will similarly be placed upon a plane (though in this case 

 there will be many different perspectives in which the 

 penny is of the same size ; when one arrangement is com- 

 pleted these will form a circle concentric with the penny), 

 and ordered as before by the apparent size of the penny. 

 By such means, all those perspectives in which the penny 

 presents a visual appearance can be arranged in a three- 

 dimensional spatial order. Experience shows that the same 

 spatial order of perspectives would have resulted if, instead 

 of the penny, we had chosen any other thing which 

 appeared in all the perspectives in question, or any other 

 method of utilising the differences between the appearances 

 of the same things in different perspectives. It is this 

 empirical fact which has made it possible to construct 

 the one all-embracing space of physics. 



The space whose construction has just been explained, 

 and whose elements are whole perspectives, will be called 

 " perspective-space." 



