12 PHYSIOLOGY CHAP. 



refer the corresponding normal peripheral excitations, as was 

 shown in the chapter on the general physiology of the nervous 

 system (Vol. III. p. 201). 



This leads us to grave philosophical questions which can only 

 be briefly touched on. How does the objectitication of sensory 

 perceptions come about ? How are we able to distinguish the 

 outer world from ourselves ? Since we do not actually feel 

 external objects, but only the Hianurs \\liidi these effect by means 

 of the sensory nerves and sense-organs in the sensory centres 

 which changes are quite different from the external objects why 

 are we convinced that our senses are not deceiving us ? These 

 problems are as important as they are hard to solve, and the 

 interpretations given to them by psychologists and physiologists 

 differ widely. 



In all ages the theory that the whole of our sensations and 

 our fundamental notions of the external world are but illusions 

 and phantasms of the mind has had many followers. Its most 

 extreme form is the absolute phenomenalism of Hume. This 

 obviously does not solve the question as to the origin of percep- 

 tions and ideas, nor does it explain the common belief in the 

 reality of the external world. 



Kant's critical idealism was a reaction from this theory. The 

 phenomena of the outrr world have nothing in common with 

 our sensations. We can know nothing about the true nature of 

 the external world : the only things we can know directly are 

 the states and phenomena of our consciousness. We can only 

 conceive of the external world by the aid of physical hypotheses 

 and speculations such as the undulatory theory, the atomistic 

 hypothesis, the mechanical theory of heat, etc. Perceptions and 

 ideas depend essentially upon congenital predispositions of the 

 senses and the brain, and on original or innate properties of 

 the mind. 



In opposition to this critical nativistic idealism is the sensory 

 empiricism which assumes that ideas are the result of observation 

 and education of the senses. Locke, Condillac, John Stuart Mill 

 deny the existence of a priori ideas. Everything comes from 

 experience or activity of the senses : the soul deprived of any 

 experience is a tabula rasa. Sensations are merely simple signs 

 representative of external objects, different indeed from them, 

 but always interpreted in the same way, from which we always 

 deduce the existence and properties of external objects by the 

 aid of previous observations. 



Helmholtz, who partially accepted this theory, recognised its 

 inadequacy to explain the facts, and assumed with Schopenhauer 

 that all our perceptions and ideas presuppose the a-priority of the 

 causal concept without which we cannot look upon objects as 

 the extrinsic cause of our sensations. This theory was further 



