THE RECEPTOR SYSTEM 205 



our sensations kinds which are absolutely distinct for our con- 

 sciousness, and not comparable mentally. We can never get con- 

 fused between a sight, a sound, and a touch, rior between pain 

 and hunger; nor can we compare them with one another: each is 

 sui generis. The fundamental difference which thus separates one 

 sensation from another is its modality. Sensations of the same 

 modality may differ; but they shade imperceptibly into one an- 

 other, and are comparable between themselves in two ways. 

 First, as regards quality: while a high and a low pitched note are 

 both auditory sensations, they are nevertheless different and yet 

 intelligibly comparable; and so are blue, purple, and red objects. 

 In the second place, sensations of the same modality are distin- 

 guishable and comparable as to amount or intensity: we readily 

 recognize and compare a loud and a weak sound of the same pitch; 

 a bright and feeble light of the same color; an acute and a slight 

 pain of the same general character. Our sensations thus differ in 

 the three aspects of modality, quality within the same modality, 

 and intensity. Certain sensations also differ in what is known as 

 the "local signs," a difference by which we tell a touch on one part 

 of the skin from a similar touch on another; or an object exciting 

 one part of the eye from an object like it, but in a different location 

 in space and exciting another part of the visual surface. 



As regards modality, we commonly distinguish five senses, those 

 of sight, sound, touch, taste, and smell; to these at least six others 

 must be added to make the list approximately complete. These addi- 

 tional senses are temperature, pain, hunger, thirst, muscle sense, 

 and equilibrium sense. The last five of this list were formerly set 

 apart as common sensations, but there seems to be no good reason 

 for viewing them in any different light from the others. 



The Psychophysical Law. Although our sensations are, in 

 modality or kind, independent of the force exciting them, they are 

 not so in degree or intensity, at least within certain limits. We 

 cannot measure the amount of a sensation and express it in foot- 

 pounds or calories, but we can get a sort of unit by determining 

 how small a difference in sensation can be perceived. This smallest 

 perceptible difference varies in the different senses and for different 

 amounts of stimulation in the same sense. Its variation in any 

 single sense follows, however, a certain law. The increase of stimu- 

 lus necessary to produce the smallest perceptible change in a sensation 



