It is scarcely necessary to remark concerning these hypotheses that they are totally destitute of foundation. A sound theory must assign real and Dot Imaginary causes for the phenomena which It professes to explain ; and such 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 images in the brain I All we can affirm with certainty respecting the means of perception is, that, under certain circum stances, that is, when an Impression is made on the bodily organs and communicated by the nerves to the brain, perception takes place. The impressions so communicated are the occasions of the mind perceiving, but we can assign no reason why it should do so under thew circum stances invariably, and not under any other, further than that such is the constitution of our nature.
If the act of perception bo 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, kc. The notion thus formed may vary in respect of distmetness in 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 is accompanied by en irresistible and immediate con section of its 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 may happen. But when it Is plainly perceived, there is, along 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 the result of a process of reasoning founded on our perceptions, but inse parably connected with them, and as 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 the belief that they exist externally to us. They do not seem to have their place in the mind itself, but to exist independently of it altogether. These statements accord with the universal experience of mankind, and may be verified by all who choose to bestow the slightest attention on the intimations of con sciousness.
It would be a tedious as well as a useless task to dwell minutely on the numerous theories that have been framed of perception. In certain important particulars nearly all of them coincide; while in others, equally if not more important, they are for the most part at variance.
Democritus taught that perception was the result of the impressions made on the organs of sense by images (elaceaa), which constantly emanated from bodies, and varied according to the conformation of their originals. (Plot. Plac. Phil.,' 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 Peripateties generally. Aristotle (' De An..' 1. iii., 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 of science, they are named intel ligible species.
The theory of Epicurus; was little other than a modification of that of Aristotle. He supposed that bodies are continually sending off from their surfaces slender films or spectres of such subtlety that they easily penetrate by the senses to the brain. (Lucret., L iv., v. 34, 46, kc.) ra Locke employe an illustration of the manner of perception that appears to have been borrowed from that of Plato :—" Methinks," he nays, "the understanding is not much unlike a closet wholly shut from light, with only some little light 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 them." 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, impreseions, for what were anciently 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 are the esclusive 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 con cerning these opinions, that they are diametrically opposed to the testimony of our own consciousness. Instead of informing its 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 be 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 arrasgcments, 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 per ceptions, such as those alluded to, is still a desideratum in this depart ment of philosophical inquiry.