210 THE PRINCIPLES OF HEREDITY 



supposed to pass from the pen to my retina, where pre- 

 sumably they set up chemical and molecular changes ; these 

 in turn set up similar changes in my optic nerve ; which 

 again set agoing other like changes in my brain. Thereupon 

 dawns in my mind a sensation which I call the " sight of the 

 pen " a sensation which is the pen to me, but which my 

 reflecting nature tells me cannot even resemble the pen. 

 The undulations of light cannot of course resemble it; 

 neither can molecular changes in the retina, the optic nerve, 

 nor the brain. Least of all can the sensations which are 

 correlated with the nervous changes that occur in the dark- 

 ness within my skull resemble it. A sensation is a mental 

 phenomenon ; a pen I suppose is a material object. No two 

 things can be more dissimilar, more utterly unlike in every 

 way than immaterial sensations and material objects. They 

 have absolutely nothing in common. I am really conscious, 

 therefore, not of the actual pen, but of something very 

 different, of a sensation which, as I suppose, is awakened in 

 me by a material object outside and beyond my mind, of the 

 real nature of which I can never by any possibility know 

 anything. 



357. I write the word " pen/' and it stands to me for a 

 sign and a symbol of the spoken word " pen." But it is only 

 a sign. It does not in the least resemble the spoken word. 

 On the other hand, the spoken word is quite unlike the 

 "seen object," of which again it is only a sign. So also the 

 sensation of sight, "the thing I see," is at most only a 

 mental symbol of a material something outside my mind, a 

 something of a nature entirely different from my mind, and 

 which I can " know " only through sensations which are part 

 of me and not of the thing they symbolize. Indeed the very- 

 existence of that "real" thing is doubtful, for I can test it 

 only by sensations with which I am encompassed as by a 

 wall, beyond which I cannot pass to the real thing, and 

 which may not, indeed cannot, tell the truth. For example, 

 I touch the pen and say it feels smooth and hard. But 

 when I mention these "qualities of the pen," I am still 

 speaking only of my own sensations, not of anything that 

 actually belongs to the pen. These qualities, these sensa- 

 tions are just as much a part of me, and as little a part of 

 the pen as the pain which is awakened if the point be driven 

 into my finger. Sometimes I know my sensations play me 

 false and symbolize things that have no real existence, as 

 when I see or touch a pen in a dream. 



358. To take another example ; I look out of my window 



