2 THE PHILOSOPHY OF BIOLOGY 



louder ; the music of the orchestra steals imperceptibly 

 on our ears and as imperceptibly fades away ; the 

 smell of the flowers lingers after we pass the shop, 

 and we do not notice just when we cease to be conscious 

 of it ; the rhythm of the ragtime continues to irritate 

 after we have ceased to hear the band — all the sense- 

 impressions that we receive melt and flow over into 

 each other and constitute our stream of consciousness, 

 and this changes from moment to moment without 

 gap or discontinuity. It is not a condition of " pure 

 sensation," but it is as nearly such as we can experience 

 in our adult intellectual life. 



It is easy to discover that many things must have 

 occurred in the street which did not affect our full 

 consciousness. We may learn afterwards that we 

 have passed several friends without recognising them ; 

 we may read in the newspapers about things that 

 happened that we might have seen, but which we did 

 not see ; we may think we know the street fairly well, 

 but we find that we have difficulty in recalling the 

 names of three contiguous shops in it ; if we happen 

 to see a photograph which was taken at the time we 

 passed through the street we are usually surprised to 

 find that there were many things there that we did 

 not see. Why is it, then, that so much that might have 

 been perceived by us was not really perceived ? We 

 cannot doubt that everything that came into the 

 visual fields of our eyes must have affected the termin- 

 ations of the optic nerves in the retinas; the complex 

 disturbances of the air in the street must have set our 

 tympanic membranes in motion ; and all the odori- 

 ferous particles inhaled into our nostrils must have 

 stimulated the olfactory mucous membranes. In all 

 these cases the stimulation of the receptor organs 

 must have initiated nervous impulses, and these must 



