INTRODUCTION 17 



which corresponds more to the viewpoint of the intro- 

 spective psychologist than to that of the physicist. The 

 attempts to explain animal conduct in terms of "trial 

 and error" or of vague "physiological states" may 

 serve as examples. None of these attempts have led or 

 can lead to any exact quantitative experiments in the 

 sense of the physicist. Other biologists have still more 

 openly adopted an anthropomorphic method of explana- 

 tion. If pleasure and pain or curiosity play a role in 

 human conduct, why should it be otherwise in animal 

 conduct? The answer to this objection is that typical 

 forced movements when produced in human beings, as, 

 e.g., in Meniere's disease or when a galvanic current goes 

 through the brain, are not accompanied by sensations of 

 pleasure or pain, and there is no reason to attribute the 

 circus movements of an animal, after lesion of the brain 

 or when one eye is blackened, to curiosity or thrills of 

 delight. An equally forcible answer lies in the fact that 

 plants show the same tropisms as animals, and it seems 

 somewhat arbitrary to assume that the bending of a plant 

 to the window or the motion of swarmspores of algae to 

 the window side of a vessel are accompanied or deter- 

 mined by curiosity or by sensations of joy or satisfaction. 

 And finally, since we know nothing of the sentiments and 

 sensations of lower animals, and are still less able to meas- 

 ure them, there is at present no place for them in science. 

 The second difficulty was created by the fact that the 

 Aristotelian viewpoint still prevails to some extent in 

 biology, namely, that an animal moves only for a pur- 

 pose, either to seek food or to seek its mate or to under- 

 take something else connected with the preservation of 

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