78 SCIENTIFIC METHOD IN PHILOSOPHY 



shine. Physiological changes also alter the appearances 

 of things. If we assume the world of common sense, 

 all these changes, including those attributed to physio- 

 logical causes, are changes in the intervening medium. 

 It is not quite so easy as in the former case to reduce this 

 set of facts to a form in which nothing is assumed beyond 

 sensible objects. Anything intervening between ourselves 

 and what we see must be invisible : our view in every 

 direction is bounded by the nearest visible object. It 

 might be objected that a dirty pane of glass, for example, 

 is visible although we can see things through it. But in 

 this case we really see a spotted patchwork : the dirtier 

 specks in the glass are visible, while the cleaner parts are 

 invisible and allow us to see what is beyond. Thus the 

 discovery that the intervening medium affects the appear- 

 ances of things cannot be made by means of the sense of 

 sight alone. 



Let us take the case of the blue spectacles, which is 

 the simplest, but may serve as a type for the others. 

 The frame of the spectacles is of course visible, but the 

 blue glass, if it is clean, is not visible. The blueness, 

 which we say is in the glass, appears as being in the 

 objects seen through the glass. The glass itself is known 

 by means of the sense of touch. In order to know that 

 it is between us and the objects seen through it, we must 

 know how to correlate the space of touch with the space 

 of sight. This correlation itself, when stated in terms 

 of the data of sense alone, is by no means a simple 

 matter. But it presents no difficulties of principle, and 

 may therefore be supposed accomplished. When it has 

 been accomplished, it becomes possible to attach a mean- 

 ing to the statement that the blue glass, which we can 

 touch, is between us and the object seen, as we say, 

 " through " it. 



