RELATION OF SENSATION TO STIMULUS 479 



certain dissolved substances with the end-organs of the gustatory nerves 

 excites in us a sensation of taste. Other substances diffused in the air 

 and carried by it to the olfactory terminations give us sensations of smell. 

 Vibrations of a certain frequency, transmitted by the air and by the auditory 

 ossicles to the endings of the auditory nerve, produce sensations of sound, 

 while light falling on the retina evokes visual sensations. 



Besides these sensations resulting from stimulation of the exteroceptive 

 system of nerves, we are aware of the existence of a number of organic 

 sensations some derived from the viscera (enteroceptive), others caused 

 by stimulation of the proprioceptive system. As examples of the latter we 

 may mention the muscular sense, by which we judge of the amount of tension 

 exerted by a contracting muscle ; the sense of position of the limbs ; and the 

 sense of position of the head, resulting from stimulation of the labyrinthine 

 organ. 



How the physiological excitatory process in nerve fibres, with its concomi- 

 tant chemical and electrical phenomena, is able on arrival at the brain to 

 excite a conscious sensation we are unable to decide, or even to discuss, since 

 we are dealing here with processes of two different orders. We should not 

 arrive any nearer to the solution of this riddle if we were able to follow out 

 the whole of the events occurring in the body as the result of the application 

 of any given stimulus to its surface. We might under these circumstances 

 be able to predict with certainty the behaviour of any animal, if we knew its 

 past history and the comparative resistance of every path in its central 

 nervous system which might possibly be traversed by any given nerve 

 impulse. Such knowledge would be purely objective and could not be used 

 to explain the ' epi-phenomenon ' of consciousness. One might in fact 

 imagine a machine which would react like a living animal, but would be 

 perfectly devoid of self -consciousness, and we should be unable in such a case 

 to decide whether consciousness were or were not present. Each one of us 

 only knows consciousness as it exists in himself. 



Before therefore we can employ our conscious sensations as a means of 

 throwing light on the conditions of action of the receptor organs of the body, 

 we must have some idea as to how far our sensations correspond to the stimuli, 

 i.e. the physical events by which they have been evoked. Appeal to our 

 own experience shows the existence of two kinds of difference between 

 various sensations. The greatest difference is found between those sensa^ 

 tions which are normally evoked by different sense-organs. Thus we are all 

 aware of the meaning attached to such qualities or such sensations as sweet, 

 red, hard, high-pitched (of sound), &c. It would be absolutely impossible 

 to compare these sensations among themselves. We cannot say, for instance, 

 that this sound is louder than that colour is red. Such fundamental differing 

 qualities of sense are spoken of as the modality of the sensation. 



On the other hand, within the sensation evoked by any one sense-organ 

 we find differences of quality which are more comparable among themselves. 

 Thus we can compare the pitch of various sounds, or the colour of various 

 objects seen with the eye. We can even say that the bitter taste of any 



