SENSATION AND ORGANS OF SPECIAL SENSE. 463 



mena as the Association of Ideas, and Memory, physio- 

 logy can give us some light; but so far as others, such as the 

 Will and the Emotions, are concerned, it has at present little 

 to offer. The phenomena of Sensation, therefore, occupy 

 at present a much larger portion of physiological works 

 than all other mental facts put together. 

 J Common Sensation and Organs of Special Sense. A 

 sensory nerve is one which, when stimulated, arouses, or 

 may arouse, a sensation in its possessor. The stimulant is 

 in all cases some form of motion, molar (e.g. mechanical 

 pressure) or molecular (as ethereal vibrations or chemical 

 changes). Since all our nerves lie within our Bodies as 

 circumscribed by the skin, and are excited within them, one 

 might a priori be inclined to suppose that the cause of all 

 sensations would appear to be within our Bodies themselves; 

 that the ihiug feU would be a modified portion of the feeler. 

 This is the case with regard to many sensations; a head- 

 ache, toothache, or earache gives us no idea of any external 

 object; it merely suggests to each of us a particular 

 state of a sensitive portion of myself. As regards many 

 sensations, however, this is not so; they suggest to us ex- 

 ternal causes, to properties of which and not to states of our 

 Bodies, we ascribe them : iiiid so they lead us to the conception 

 of an external universe. A knife laid on the skin produces 

 changes in it which lead us to think not of a state of our 

 skin, but of states of some object outside the skin; we 

 believe we feel a cold heavy hard thing in contact with it. 

 Nevertheless we have no sensory nerves going into the knife 

 and informing us directly of its condition; what we really 

 feel are the modifications of our Body produced by it r 

 although we irresistibly think of them as properties of the 

 knife of some object that is no part of our Body, and not as; 

 states of the latter itself. Let now the knife cut through 

 the skin; we feel no more knife, but experience pain, 

 which we think of as a condition of ourselves. We do not 

 say the knife is painful, but that our finger is, and yet we 

 have, so far as sensation goes, as much reason to call the 

 knife painful as cold. Applied one way it produced local 

 changes arousing a sensation of cold, and in another local 



