THE WORLD OF LIVING ORGANISMS 135 



impressions on the animal, and provoke it to different re- 

 sponses. Likewise he will assume that the same stimulus 

 makes the same impression on the animal, and calls forth the 

 same response. 



Neither of these assumptions is correct. As we have 

 seen, different stimuli, proceeding from the most various 

 objects, produce the same response in Paramecium. And 

 the same stimulus — an air- wave, for instance — produces a 

 different effect when it strikes the skin of a man from what it 

 does when it reaches his ear : in the one case, we speak of 

 vibration, in the other, of sound. 



For the biological study of an animal, therefore, knowledge 

 of the stimuli is not enough. Yet another factor must be 

 sought in order to explain why the animal should give the 

 same response. I shall call this factor an indication. 



The indication is not a physiological factor like the ^ 

 stimulus, but a biological factor which we deduce from the 

 animal's response. It cannot, however, be constructed from 

 the stimuli alone, because its formation depends on the animal 

 itself, and because it cannot be understood at all without 

 knowledge of the means that the animal employs for that 

 formation. 



As soon as we are dealing with the inner processes of the 

 animal, the psychologist comes along all ready with his asser- 

 tion that we must take account of the psychic qualities. 

 Again we must insist that the biologist, like the physiologist 

 and the physicist, cannot admit such a change of stand- 

 point without deserting his science. This does not mean, 

 however, that he should give up the critical judgment which 

 should be his in virtue of his position as observer. In this 

 sense, the biologist is also a psychologist, because any event 

 that comes to his knowledge takes place in his world, and 

 that world is built up of his subjective sensations. 



It is not possible even for the biologist to transfer the 



