Mind and Motion and Monism | Page 6

of the stimulus.' Thus, for instance, if 1,000 candles are all throwing their light upon the same screen, we should require ten more candles to be added before our eyes could perceive any difference in the amount of illumination. But if we begin with only 100 candles shining upon the screen, we should perceive an increase in the illumination by adding a single candle. And what is true of sight is equally true of all the other senses: if any stimulus is increased, the smallest increase of sensation first occurs when the stimulus rises one per cent, above its original intensity. Such being the law on the side of sensation, suppose that we place upon the optic nerve of an animal the wires proceeding from a delicate galvanometer, we find that every time we stimulate the eye with light, the needle of the galvanometer moves, showing electrical changes going on in the nerve, caused by the molecular agitations. Now these electrical changes are found to vary in intensity with the intensity of the light used as a stimulus, and they do so very nearly in accordance with the law of sensation just mentioned. So we say that in sensation the cerebral hemispheres are, as it were, acting the part of galvanometers in appreciating the amount of molecular change which is going on in sensory nerves; and that they record their readings in the mind as faithfully as a galvanometer records its readings on the dial.
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Hitherto we have been considering certain features in the physiology of nervous action, so far as this can be appreciated by means of physiological instruments. But we have just seen that the cerebral hemispheres may themselves be regarded as such instruments, which record in our minds their readings of changes going on in our nerves. Hence, when other physiological instruments fail us, we may gain much additional insight touching the movements of nervous matter by attending to the thoughts and feelings of our own minds; for these are so many indices of what is going on in the cerebral hemispheres. I therefore propose next to contemplate the mind, considered thus as a physiological instrument.
The same scientific instinct which led Hobbes so truly to anticipate the progress of physiology, led him not less truly to anticipate the progress of psychology. For just as he was the first to enunciate the fundamental principle of nerve-action in the vibration of molecules, so was he likewise the first to enunciate the fundamental principle of psychology in the association of ideas. And the great advance of knowledge which has been made since his day with respect to both these principles, entitles us to be much more confident than even he was that they are in some way intimately united. Moreover, the manner in which they are so united we have begun clearly to understand. For we know from our study of nerve-action in general, that when once a wave of invisible or molecular movement passes through any line of nerve-structure, it leaves behind it a change in the structure such that it is afterwards more easy for a similar wave, when started from the same point, to pursue the same course. Or, to adopt a simile from Hobbes, just as water upon a table flows most readily in the lines which have been wetted by a previous flow, so the invisible waves of nerve-action pass most readily in the lines of a previous passage. This is the reason why in any exercise requiring muscular co-ordination, or dexterity, 'practice makes perfect:' the nerve-centres concerned learn to perform their work by frequently repeating it, because in this way the needful lines of wave-movement in the structure of the nerve-centre are rendered more and more permeable by use. Now we have seen that in the nerve-centres called the cerebral hemispheres, wave-movement of this kind is accompanied with feeling. Changes of consciousness follow step by step these waves of movement in the brain, and therefore when on two successive occasions the waves of movement pursue the same pathway in the brain, they are attended with a succession of the same ideas in the mind. Thus we see that the tendency of ideas to recur in the same order as that in which they have previously occurred, is merely an obverse expression of the fact that lines of wave-movement in the brain become more and more permeable by use. So it comes that a child can learn its lessons by frequently repeating them; so it is that all our knowledge is accumulated; and so it is that all our thinking is conducted.
A wholly new field of inquiry is thus opened up. By using our own consciousness as a physiological instrument of the greatest delicacy, we are able to learn a great deal