The RelativitT/ of Knowledge. 33 



to accuracy the ideas of other minds. Why ? Because 

 their ideas and ours are both of a kind they are both 

 ideas. We cannot so picture the actual state of that exter- 

 nal reality which produces in us the sensation of a revolv- 

 ing wheel. A sharp knife will produce the sensation of 

 pain. Have we any reason for believing that the pain thus 

 produced was part of the knife-blade, that got into us from 

 it ? A flea moving over our backs will give an itching sen- 

 sation. Does the sensation, <'itch," escape from the feet 

 of the flea and enter the back ? A gentle touch of the 

 ribs by mischievous finger-tips will produce the sensation 

 of tickle. Is the '' tickle " first in the finger-tips and then 

 in the ribs ? An electric induction-coil produces a sensa- 

 tion of shock. Is the shock first in the coil and then in 

 the person shocked ? Our judgments demand that these 

 questions be answered in the negative. W^hatever stimu- 

 lates the nerve-centres susceptible of itching, pain, or tickle, 

 will give us these sensations ; and the stimulating thing 

 need in no sense resemble the sensation produced. 



Now, it is precisely so with the nerve-centre which appre- 

 hends motion. Whatever stimulates it, makes motion vis- 

 ible to our senses. The poisonous ptomaines of the blood in 

 the delirium of fever produce just such stimulation. Drugs 

 can do the same. An undigested supper will make us see 

 bodies in motion, or believe we move ourselves, in dreams. 

 As the fingers tickle the ribs, so drugs and poisons tickle 

 the brain into seeing moving bodies. They evoke the sen- 

 sations of space and motion. The space and motion I am 

 conscious of in a dream is, so far as my sensations are con- 

 cerned, the same space and motion of my normal, waking 

 moments. Surely you will not assert that the dream-space 

 and dream-motion, and a badly digested supper that tickled 

 it into being, are identical ? If a dose of rich pie-crust 

 can make the sensations of space and motion appear in 

 consciousness, what evidence have we that the actual things 

 in themselves, that encompass us within Nature, are any 

 more like these sensations than the pie-crust is ? If alcohol 

 in excess can tickle our nerve-centres into seeing moving 

 snakes, how do we know that the real snakes are not as 

 unlike these sensations which they produce as the alcohol 

 is ? We can believe that our sensations of space and mo- 

 tion, and the things normally causing them to appear, are 

 just alike. We can also, if we wish, believe that mis- 



