500 THE HUMAN BODY. 



ever (and then but approximately), for sensations of medium 

 intensity; it is applicable, for example, to light sensations of 

 all degrees between those aroused by the light of a candle 

 and ordinary clear daylight : but it is not true for luminosi- 

 ties so feeble as only to be seen at all with difficulty, or so 

 bright as to be dazzling. 



Besides their variations in intensity, dependent on varia- 

 tions in the strength of the stimulus, our sensations also vary 

 with the irritability of the sensory apparatus itself; which is 

 not constant from time to time or from person to person. 

 In the above statements the condition of the sense-organ and 

 its nervous connections is presumed to remain the same 

 throughout. 



Perceptions. In every sensation we have to carefully 

 distinguish between the pure sensation and certain judg- 

 ments founded upon it; we have to distinguish between what 

 we really feel and what we think we feel; and very often 

 firmly believe we do feel when we do not. 



The most important of these judgments is that which 

 leads us to ascribe certain sensations, those aroused through 

 organs of special sense, to external objects that outer refer- 

 ence of our sensations which leads us to form ideas concern- 

 ing the existence, form, position, and properties of external 

 things. Such representations as these, founded on our senses, 

 are called perceptions. Since these always imply some 

 mental activity in addition to a mere feeling, their full dis- 

 cussion belongs to the domain of Psychology. Physiology, 

 however, is concerned with them so far as it can determine 

 the conditions of stimulation and neurosis under which a 

 given mental representation concerning a sensation is made. 

 It is quite certain that we can feel nothing but states of our- 

 selves, but, as already pointed out, we have no hesitation in 

 saying we feel a hard or a cold, a rough or smooth body. 

 When we look at a distant object we usually make no demur 

 to saying that we perceive it. What we really feel is, how- 

 ever, the change produced by it in our eyes. There are no 

 parts of our Bodies reaching to a tree or a house a mile off 

 and yet we seem to feel all the while that we are looking at 

 the tree or the house and feeling them, and not merely ex- 

 periencing modifications of our own eyes or brains. When 

 reading we feel that what we really see is the book; and yet 



