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PERCEPTION (from Lat. percipere, to perceive), is that power or actof the mind by which it holds communication with the external world. It is distinguished from conception by the circumstance that its objects arc in every instanco supposed to have an actual existence. We may conceive things that have no reality, but we arc never said to perceive such things. Perception differs from consciousness in that it takes cognisance only of objects without the mind. We perceive a man, a horse, a tree: when we think or fed, we are conscious of our thoughts and emotions. It is further supposed in perception that the objects of it are present. We can remember former objects of perception, but we do not perceive them again until they are once more present. Besides the sense which has been explained, the term perception is sometimes analogically employed in common speech in reference to truths the evidence of which is certain. Thus we may perceive the truth of a mathematical proposition. But Mr. Hume is perhaps the only writer of eminence who designedly applies the word in a metaphysical disquisition with a meaning different from that which has been here assigned to it By him it is applied indiscriminately to all the operations and states of the mind; passions being designated perceptions, and the acts of memory and imagination converted into so many nets of perception. Such latitude of phraseology confounds under one general name things essentially distinct, and tends to introduce vagueness and inaccuracy into a department of philosophical investigation where definitencss and precision arc peculiarly indispensable.

The distinction between things perceived (oitrdqra) and things conceived (voijro) was familial- to the Greek philosophers and to tbeir Latin expositors, of whom Cicero expresses the former class of things by the phrases ' qua? sunt,'' qusB cerui tangivo possum;' and the latter by the phrases 'quse tangi demonstrative non possunt, cerni tamen animo atque intelligi possunt,' and gives examples of each. (Top., v.)

The perceptive faculty is exercised through the instrumentality of the senses. Wo see by means of the eye, and Lear by means of the ear, and so in reference to the other senses. An individual in whom these organs are wanting or defective, will cither not perceive at all, or perceive imperfectly. In order to perception it is requisite that an impression should be made on the organ of sense, either by the direct application of the object, or through some medium that communicates with the object and the organ. Thus an immediato application is necessary with regard to the senses of taste and touch ; but only an intermediate one with regard to those of sight, hearing, and smell. The impression made on the organs of sense affects the nerves, and is by them conveyed to the brain. The necessity of this communication is ascertained by observation. If the nerve appropriated to any organ be cut or tied hard, no perception takes place; and the same result is noticed in certain disordeied conditions of I ho brain, even though the organs of sen«o and the nerves perform their respective functions. When however the conditions that have been specified are complied with, perception ensues.

Various theories bare been formed to explain the functions of the nerves and brain in connection with perception. It was imagined by the antients that the nervous fibres saw tubular, and filled with a subtile vapour named animal spirits; that the brain is a gland by which this aithercal fluid is secreted ; and that by means of it the nerves perform their office. (Rcid, Essay ii., ch. 3.) Des Cartes, who adonted this hypothesis, has described with great minuteness how all mental operations and movements are accompl^hed through the agency referred to. Dr. Briggs, Newton's instructor in anatomy, was the first who proposed a new doetrine on this point. He maintained that the nerves operate by vibrations, like musical chords, and thus conduct impressions to the brain. Newton himself (Opt., qo_ 23) appears to have been inclined to a notion of this kind, and the suggestions relating to it thrown out by him as a query were afterwards amplified and defended by Hartley. The latter supposed that' external objects impressed on the senses occasion, first in the nerves on which they are impressed and then in the brain, vibrations of the smaL and, as one may say, infinitesimal medullary particles ;* and that these vibrations are excited, propagated, and kept up partly by the ether, that is, by a very subtile elastic fluid, partly by the uniformity, continuity, softness, and active powers of the medullary substance of the brain, spinal marrow, and nerves.' (Observations on Man, part i, prop 4. 5.) Both Des Cartes and Hartley believed that by the action of the nerves in the manner described by them, images of external objects were formed in the brain.

It is scarcely necessary to remark concerning these hypotheses that they are totally destitute of foundation. A sound theory must assign real and not imaginary cause* for the phenomena which it professes to explain; and sudi causes must have a manifest competency to the effects ascribed to them. But the hypotheses in question entirely want both of these essentials. Who can prove the existence of the animal spirits of Des Cartes, or the vibrations of Hartley; or, granting their existence, who can show any correspondence between them and the formation of imagt* in the brain? All we can affirm with certainty respecting the means of perception is, that, under certain circumstances, that is, when an impression is made on the bodily organ, and communicated by the nerves to the brain, perception takes place. The impressions so communicated are tho occasions of the mind perceiving, but wc can assign no reason why it should do so under these circumstances invariably, and not under any other, further than that such u the constitution of our nature.

If the act of perception be examined, it will be found that we obtain by it a certain amount of information respecting the object perceived. We discover that it has particular qualities, as for example, that it is extended, that it has figure, that it is hard or soft, rough or smooth. &e. The notion thus formed may vary in respect of distinctness la all possible degrees. In the light of twilight a body is discerned more obscurely than in the full light of noon day: and more obscurely still in proportion as the darkness deepens. The notion we get of an object by perception a afto.npanied by an irresistible and immediate conviction nf iu real existence. An object may indeed be perceived so indistinctly as to leave us in doubt whether it be real or not If it be very distant, or involved in darkness, this m=T happen. But when it is plainly perceived, there is, slung with the perception, a perfect conviction of its reality. We can no more doubt of its existence than we can of onr own. And this conviction is immediate. It is not tho result of a process of reasoning founded on our perceptions, but inseparably connected with them, and a} instantaneous as the assent rendered to axiomatic truths. It may be also remarked that the belief in the existence of the objects of perception is not more immediate and deeply rooted than is lb* belief that they exist externally to us. They do not »«=i to havo their place in the mind itself, but to exist indepfc dently of it altogether. These statements accord with toe universal experience of mankind, and may be verified by all who choose to bestow the slightest attention on the intima tions of consciousness.

It would be n tedious as wdl as a useless task to dwell minutely on the numerous theories that have been framed of perception. In certain ir—-—**nt particulars nearly ail of them coincide; while f~ -sally if not more important, they are for the "inee. Dcmocritus taught 'he result of the impressions made on t usages tiTfsfiaj. waicli constantly emanated from bodies, and varied according to the conformation of their originals. (Plut., Plac, Pul., 1. iv., ch. 8, &c.)

Plato, in the seventh book of his'Republic'(ad init.) illustrates the manner in which we perceive objects, by the figure of a cave, in which men lie bound, so that they can turn their eyes only to one part of it, where rays from a distant light stream in, and shadows of bodies, supposed to pass between them and the light, are beheld, the bodies themselves being invisible. He thus conceived that we perceive only the shadows of things, and not things themselves. This opinion of Plato was substantially the same with that of his scholar Aristotle, and of the Peripatetics generally. Aristotle (De An, 1. hi., c. 2, 3) taught that as the senses cannot receive material objects themselves, they receive their images. These images are the only objects of perception to the mind. As impressed upon the senses, they are termed sensible species; more spiritualised, they become objects of memory and imagination, and are termed phantasms; still further refined, so as to be objects uf science, they are named intelligible species.

The theory of Epicurus was little other than a modification of that of Aristotle. He supposed that bodies are continually Fending off from their surfaces slender films or spectres of such subtlety that they easily penetrate by the senses to the brain. (Lucret., 1. iv., v. 34, 46, Stc.)

Locke employs an illustration of the manner of perception that appears to have been borrowed from that of Plato:— 'Methinks,' he says, 'the understanding is not much unlike a closet wholly shut from light, with only some little opening left to let in external visible resemblances, or ideas of things without. Would the pictures coming into a dark room but stay there, and be so orderly as to be found upon occasion, it would very much resemble the understanding of a man in reference to all objects of sight and the ideas of thera.'

The similitude of Locke, or rather of Plato, may be applied to all the systems of perception that have ever been formed, by merely substituting ideas, and. in the case of Hume, impressions, for what were antiently denominated species and phantasms. All these theories agree in maintaining that images are the only immediate objects of perception to the mind. Hume, Berkeley, and others indeed hold that these aro the exclusive objects; but the common hypothesis admits the existence of things of which these are but the representatives, and which we mediately discern. It may be sufficient to remark concerning these opinions, that they are diametrically opposed to the testimony of our own consciousness. Instead of informing us that images alone are the direct objects of our perception, consciousness intimates nothing respecting images at all. Unless its representations are altogether deceptive, it is not things within the mind, but things external to it, that we perceive; not images of objects, but the very objects themselves. This is testimony to which we yield instinctive credence. It is too cogent and unquestionable to be set aside by reasoning of any kind, far less by reasoning based upon certain imagined relations subsisting between matter and spirit which we are incapable of apprehending, and the application to mind of laws which apply solely to the objects of physical investigation. One observation, intentionally deferred, remains still to he made respecting perception, namely, that it is greatly modified by habit and by the cultivation and development of the other powers. Thus the perceptions of a man and those of a child, both contemplating a piece of complex machinery, the one being aware of its principles and arrangements, the other completely ignorant of them, must in some respects considerably differ. In like manner the perceptions of a blind man, by means of those organs of sense which are unimpaired, are distinguished in many particulars from those of the individual who has never been without the faculty of vision. Numerous instances of a similar kind might easily be specified. A full account of acquired perceptions, such as those alluded to, is stilt a desideratum in this department of philosophical inquiry

Note - this article incorporates content from The Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge (1840)

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