THE RELATIVITY OF KNOWLEDGE 



In the second place, it is extremely probable 

 that the subjective feelings awakened by the 

 same external cause are not quite alike in any 

 two individuals of the same species. In those 

 persons who are troubled with Daltonism, or 

 colour-blindness, luminous undulations so dif- 

 ferent as those of red and green awaken feelings 

 that are identical. On the other hand, " aerial 

 pulses recurring at the rate of i6 per second are 

 perceived by some as separate pulses ; but by 

 some they are perceived as a tone of very low 

 pitch. Similarly at the other extreme. Vibra- 

 tions exceeding 30,000 per second are inaudi- 

 ble through certain ears ; while through ears 

 that are, as we may suppose, of somewhat un- 

 like structures, these rapid vibrations are known 

 as an excessively acute sound." ^ 



And thirdly, let us notice a set of facts which 

 are so familiar to us that we overlook their sig- 

 nificance. " A whiff of ammonia, coming in 

 contact with the eyes, produces a smart ; getting 

 into the nostrils, excites the consciousness we 

 describe as an intolerably strong odour; being 



^ *«It is probable that the antennae of insects respond to 

 stimuli which leave us insensible, while stimuli which affect 

 us leave them undisturbed. . . . We know there are a thou- 

 sand tremours in the air which beat upon our ears unheard ; 

 and if more sensitive organs are capable of hearing some of 

 these, there must be tremours which no organism can feel.'* 

 — Lewes, Problems of Life and Mind, vol. i. p. 255. 



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