Another limit to our sensibility, which needs to be just touched on here, is known by the name of the specific energy of the nerves.  One and the same nerve-fibre always reacts in a precisely similar way, whatever the nature of the stimulus.  Thus, when the optic nerve is stimulated in any manner, whether by light, mechanical pressure, or an electric current, the same effect, a sensation of light, follows.[27] In a usual way, a given class of nerve-fibre is only stimulated by one kind of stimulus.  Thus, the retina, in ordinary circumstances, is stimulated by light.  Owing to this fact, there has arisen a deeply organized habit of translating the impression in one particular way.  Thus, I instinctively regard a sensation received by means of the optic nerve as one caused by light.

Accordingly, whenever circumstances arise in which a like sensation is produced by another kind of stimulus, we fall into illusion.  The phosphenes, or circles of light which are seen when the hinder part of the eyeball is pressed, may be said to be illusory in so far as we speak of them as perceptions of light, thus referring them to the external physical agency which usually causes them.  The same remark applies to those “subjective sensations,” as they are called, which are known to have as their physical cause subjective stimuli, consisting, in the case of sight, in varying conditions of the peripheral organ, as increased blood-pressure.  Strictly speaking, such simple feelings as these appear to be, involve an ingredient of false perception:  in saying that we perceive light at all, we go beyond the pure sensation, interpreting this wrongly.

Very closely connected with this limitation of our sensibility is another which refers to the consciousness of the local seat, or origin of the impression.  This has so far its basis in the sensation itself as it is well known that (within the limits of local discrimination, referred to above) sensations have a particular “local” colour, which varies in the case of each of the nervous fibres by the stimulation of which they arise.[28] But though this much is known through a difference in the sensibility, nothing more is known.  Nothing can certainly be ascertained by a mere inspection of the sensation as to the distance the nervous process has travelled, whether from the peripheral termination of the fibre or from some intermediate point.

In a general way, we refer our sensations to the peripheral endings of the nerves concerned, according to what physiologists have called “the law of eccentricity.”  Thus I am said to feel the pain caused by a bruise in the foot in the member itself.  This applies also to some of the sensations of the special senses.  Thus, impressions of taste are clearly localized in the corresponding peripheral terminations.