Each afferent nerve comes from a determinate part of the periphery and is played upon and excited to its inward activity by a particular force of the outer world. Usually it is insensible to other forces: thus the optic nerves are not impressible by air-waves, nor those of the skin by lightwaves. The lingual nerve is not excited by aromatic effluvia, the auditory nerve is unaffected by heat. Each selects from the vibrations of the outer world some one rate to which it responds exclusively. The result is that our sensations form a discontinuous series, broken by enormous gaps. There is no reason to suppose that the order of vibrations in the outer world is anything like as interrupted as the order of our sensations. Between the quickest audible air-waves (40,000 vibrations a second at the outside) and the slowest sensible heat-waves (which number probably billions), Nature must somewhere have realized innumerable intermediary rates which we have no nerves for perceiving. The process in the nerve-fibres themselves is very likely the same, or much the same, in all the different nerves. It is the so-called 'current'; but the current is started by one order of outer vibrations in the retina, and in the ear, for example, by another. This is due to the different terminal organs with which the several afferent nerves are armed. Just as we arm ourselves with a spoon to pick up soup, and with a fork to pick up meat, so our nerve-fibres arm themselves with one sort of endapparatus to pick up air-waves, with another to pick up ether-waves. The terminal apparatus always consists of modified epithelial cells with which the fibre is continuous. The fibre itself is not directly excitable by the outer agent which impresses the terminal organ. The optic fibres are unmoved by the direct rays of the sun; a cutaneous nervetrunk may be touched with ice without feeling cold.* The

*The subject may feel pain, however, in this experiment; and it must be admitted that nerve-fibres of every description, terminal organs as well, are to some degree excitable by mechanical violence and by the electric current.

fibres are mere transmitters; the terminal organs are so many imperfect telephones into which the material world speaks, and each of which takes up but a portion of what it says; the brain-cells at the fibres' central end are as many others at which the mind listens to the far-off call.

The 'Specific Energies' of the Various Parts of the Brain.—To a certain extent anatomists have traced definitely the paths which the sensory nerve-fibres follow after their entrance into the centres, as far as their termination in the gray matter of the cerebral convolutions.* It will be shown on a later page that the consciousness which accompanies the excitement of this gray matter varies from one portion of it to another. It is consciousness of things seen, when the occipital lobes, and of things heard, when the upper part of the temporal lobes, share in the excitement. Each region of the cerebral cortex responds to the stimulation which its afferent fibres bring to it, in a manner with which a peculiar quality of feeling seems invariably correlated. This is what has been called the law of 'specific energies' in the nervous system. Of course we are without even a conjectural explanation of the ground of such a law. Psychologists (as Lewes, Wundt, Rosenthal, Goldscheider, etc.) have debated a good deal as to whether the specific quality of the feeling depends solely on the place stimulated in the cortex, or on the sort of current which the nerve pours in. Doubtless the sort of outer force habitually impinging on the end-organ gradually modifies the end-organ, the sort of commotion received from the endorgan modifies the fibre, and the sort of current a so-modified fibre pours into the cortical centre modifies the centre. The modification of the centre in turn (though no man

* Thus the optic nerve-fibres are traced to the occipital lobes, the olfactory tracts go to the lower part of the temporal lobe (hippocampal convolution), the auditory nerve-fibres pass first to the cerebellum, and probably from thence to the upper part of the temporal lobe. These anatomical terms used in this chapter will be explained later, The cortex is the gray surface of the convolutions.

can guess how or why) seems to modify the resultant consciousness. But these adaptive modifications must be excessively slow; and as matters actually stand in any adult individual, it is safe to say that, more than anything else, the place excited in his cortex decides what kind of thing he shall feel. Whether we press the retina, or prick, cut, pinch, or galvanize the living optic nerve, the Subject always feels flashes of light, since the ultimate result of our operations is to stimulate the cortex of his occipital region. Our habitual ways of feeling outer things thus depend on which convolutions happen to be connected with the particular end-organs which those things impress. We see the sunshine and the fire, simply because the only peripheral end-organ susceptible of taking up the ether-waves which these objects radiate excites those particular fibres which run to the centres of sight. If we could interchange the inward connections, we should feel the world in altogether new ways. If, for instance, we could splice the outer extremity of our optic nerves to our ears, and that of our auditory nerves to our eyes, we should hear the lightning and see the thunder, see the symphony and hear the conductor's movements. Such hypotheses as these form a good training for neophytes in the idealistic philosophy!

Sensation distinguished from Perception. It is impossible rigorously to define a sensation; and in the actual life of consciousness sensations, popularly so called, and perceptions merge into each other by insensible degrees. All we can say is that what we mean by sensations are FIRST things in the way of consciousness. They are the immediate results upon consciousness of nerve-currents as they enter the brain, and before they have awakened any suggestions or associations with past experience. But it is obvious that such immediate sensations can only be realized in the earliest days of life. They are all but impossible to adults with memories and stores of associations acquired. Prior to all impressions on sense-organs, the brain is plunged in deep sleep and consciousness is practically non

existent. Even the first weeks after birth are passed in almost unbroken sleep by human infants. It takes a strong message from the sense-organs to break this slumber. In a new-born brain this gives rise to an absolutely pure sensation. But the experience leaves its unimaginable touch' on the matter of the convolutions, and the next impression which a sense-organ transmits produces a cerebral reaction in which the awakened vestige of the last impression plays its part. Another sort of feeling and a higher grade of cognition are the consequence. 'Ideas' about the object mingle with the awareness of its mere sensible presence, we name it, class it, compare it, utter propositions concerning it, and the complication of the possible consciousness which an incoming current may arouse, goes on increasing to the end of life. In general, this higher consciousness about things is called Perception, the mere inarticulate feeling of their presence is Sensation, so far as we have it at all. To some degree we seem able to lapse into this inarticulate feeling at moments when our attention is entirely dispersed.

Sensations are cognitive. A sensation is thus an abstraction seldom realized by itself; and the object which a sensation knows is an abstract object which cannot exist alone. 'Sensible qualities' are the objects of sensation. The sensations of the eye are aware of the colors of things, those of the ear are acquainted with their sounds; those of the skin feel their tangible heaviness, sharpness, warmth or coldness, etc., etc. From all the organs of the body currents may come which reveal to us the quality of pain, and to a certain extent that of pleasure.

Such qualities as stickiness, roughness, etc., are supposed to be felt through the coöperation of muscular sensations with those of the skin. The geometrical qualities of things, on the other hand, their shapes, bignesses, distances, etc. (so far as we discriminate and identify them), are by most psychologists supposed to be impossible without the evocation of memories from the past; and the

cognition of these attributes is thus considered to exceed the power of sensation pure and simple.

'Knowledge of Acquaintance' and 'Knowledge about.'Sensation, thus considered, differs from perception only in the extreme simplicity of its object or content. Its object, being a simple quality, is sensibly homogeneous; and its function is that of mere acquaintance with this homogeneous seeming fact. Perception's function, on the other hand, is that of knowing something about the fact.

we must know what fact we mean, all the while, and the various whats are what sensations give. Our earliest thoughts are almost exclusively sensational. They give us a set of whats, or thats, or its; of subjects of discourse in other words, with their relations not yet brought out. The first time we see light, in Condillac's phrase we are it rather than see it. But all our later optical knowledge is about what this experience gives. And though we were struck blind from that first moment, our scholarship in the subject would lack no essential feature so long as our memory remained. In training-institutions for the blind they teach the pupils as much about light as in ordinary schools. Reflection, refraction, the spectrum, the ether-theory, etc., are all studied. But the best taught born-blind pupil of such an establishment yet lacks a knowledge which the least instructed seeing baby has. They can never show him what light is in its first intention'; and the loss of that sensible knowledge no book-learning can replace. All this is so obvious that we usually find sensation 'postulated' as an element of experience, even by those philosophers who are least inclined to make much of its importance, or to pay respect to the knowledge which it brings.

Sensations distinguished from Images.-Both sensation and perception, for all their difference, are yet alike in that their objects appear vivid, lively, and present. Objects merely thought of, recollected, or imagined, on the contrary, are relatively faint and devoid of this pungency, or tang, this quality of real presence which the objects of sensation