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at maturity without knowing the existence of a female of his own species, might feel it, as a new-born child feels the want of food, without having any determinate notion of what was proper to gratify it.
6. Beauty of form and colour, which act, in these cases, through the medium of the imagination only, have nothing to do with this mere irritation of the nerves, whether it proceed from internal or external stimuli; for this irritability extends in some instances to vegetable substances, which have no power of perception; but of which the organic parts are not only irritable, but require the touch of an insect or other extraneous body to render them effective in reproduction.
7. Many sorts of plants seem, in other respects, capable of sensation, as far as this power consists in the mere aptitude of the organs to receive impressions: but it does not appear that the impressions ever go further than the organs, which receive them; and if they do not, it is evident that they can excite neither pleasure nor pain; nor leave any traces or memorials behind them of any kind. The impressions, therefore, being unperceived, produce only mechanical vibrations in the fibres, of which the sufferer is not conscious, and which, therefore, only differ in their cause or mode from those which impulse or attraction
excites in the component parts of metals: for though the impressions upon the external organs of sense are the primary causes of those sensations, which imprint the ideas of them upon the mind; yet the perception of those sensations, and consequently the pleasures and pains arising from them, as well as the ideas which they imprint, are in the brain; from which, if the organ be separated, though it may retain its irritability, and its apparent sensibility, for a considerable time, it will still be utterly incapable of sensation, and in exactly the same predicament as we have supposed the irritable organs of vegetables to be*. On the contrary, sensations, exactly resembling those produced by impressions on the external organs, will continue to be felt when the organ is no more; it being common for a person, who has lost a limb, to imagine that he feels a pain in the extremity which has been ampu
* I am speaking only of animals whose organization is perfect, and analogous to cur own. I know that butterflies, wasps, &c. do appear to be sensible of pleasure or pain, and even live and linger for a long time after their heads are off; but then it does not appear that the heads of such animals contain any centre of organization or seat of life analogous to the brain in birds and quadrupeds. Many of the cold-blooded amphibious animals also retain life for a long time after the head has been separated from the body; but if there be any sense of pain left, I should conceive it to be in the head only.
tated; that is, really to feel a pain, excited by some internal cause, similar to that which he had before felt in that extremity.
8. For this, as well as for many other reasons, it is evident that neither the sensations, nor the ideas imprinted by them, have any resemblance to the objects, or the qualities of objects, which have produced them; but that the connection between them, howsoever spontaneous and immediate it may seem, is merely habitual, and the result of experience and observation *. Certain sensations constantly accompanying certain objects, we naturally and justly conceive those objects to be the cause of them; and when impressions are made upon two or more different organs, by the same object, at the same time, the evidence of their being so is as strong and certain as any, that does not admit of demonstration, by comparative numbers and quantities, can be. I may have a pain in my hand, produced
. by some internal cause, so exactly resembling that produced by the puncture of a needle, or
• See Locke, Berkeley, Hume, Reid, &c. Locke, indeed, with some hesitation excepted what he calls the primary qualities of bodies, such as figure, extension, &c. and admitted that the ideas of these were resemblances of them, (Essay on Haman Understanding, Book II. c. viii.) but Berkeley and Hume found no difficulty in confuting him, and proving that these had no more similitude to their archetypes than any others,
the burning of a caustic, that, if I had no other sense, but that of feeling, I might not be able to distinguish the one from the other : but if I see the needle thrust into it, or the caustic applied to it, and feel the pain to commence at the same instant, I naturally connect
I them as cause and effect; and, having once imprinted them as such in my memory, continue to connect them ever afterwards. Neither the needle, however, nor the puncture; the caustic, nor the burning, have any resemblance, either with the sensations felt or with the remembrances of them imprinted: but the evidence of two senses to one point becomes that of a parallax *; and the force of it is
A parallax in astronomy is the difference between the relative situation of any heavenly body, as it is seen from the surface, and as it would be seen from the centre of the earth : which difference, being ascertained by an angle, the base of which is half the earth's diametre, affords evidence of the real magnitude and distance of the body, to which the perpendicular of that angle extends.
The term, though usually employed in astronomy, more properly belongs to optics; and may be equally applied to any visible object, which, by a variation of the point of sight, appears to vary its relative situation; and the extent of such variation, being ascertained by similar means, will afford similar evidence concerning the object.
The author should feel shame in thus obtruding explanations, which, to every reader of liberal education, must appear useless and impertinent, had not a whole synod of professed critics proclaimed their want of them, by petuD 3
doubled with every repetition of the same sen
sation from the same external cause. Of Touch.:
9. These remembrances, or retained perceptions or notions, Des Cartes and Locke called ideas; a name borrowed from the Platonic philosophy, with which their followers Berkeley and Hume contrived to subvert first the material, and then the intellectual world. Plato, indeed, had before attempted to subvert the former; or, at least, to render its foundations very insecure: for he too perceived that there was no resemblance between ideas and the material objects that they appear to represent in the mind: but concluding that these notions must be exact copies from some real existences, he derived them from the intellectual world; whence the human soul sprang, and where the eternal ideas, according to which the fleeting and changeable forms, which we see impressed upon gross matter, remained immutable in the divine mind. All real knowledge, therefore, according to this philosopher, was innate; and the improvement of it consisted in recovering and restoring the images, with which the soul had originally been endowed, but which were buried and obscured in the opaque dross of matter. These images or ideas were not derived from lantly reproaching him with their own ignorance. EDINB. REVIEW, No. XIV.