are enabled by the eye to cognize the solidity as well as the extension of objects. The difference of figure in the two images gives us this. That such is the case is shown by an instrument, the stereoscope, so constructed as to present separately the image as formed on each eye, which, when separately viewed, appear as mere plane surfaces, but when viewed together, the right image with the right eye, and the left one with the left eye, at the same time, present no longer the appearance of plane surfaces, but the two images combine to form one distinct figure, and that a solid, having length, breadth, thickness, and standing out with all the semblance of the real object.
It is hardly necessary to say that if extension is an object of perception by the eye, so also is figure, which is merely the limitation of extension in different directions.
Second Question—Does Sight give Distance?—Is it also by vision that we obtain the idea of the distance of objects and their externality? Does vision alone give the idea that what we see is numerically distinct from ourselves, and that it occupies this or that particular locality? So it would seem, judging from the impression left upon the mind in the act of vision. We seem to see the object as here or there, external, more or less distant in space. We distinguish it from ourselves.
The negative View.—This is denied by some. All that we see, they contend, is merely the light coming from the object, and from the variations and modifications which this exhibits we learn to judge by experience of the distance and locality of the object. It is a matter of judgment and not of perception. We have learned to associate the two things, the visual appearance and the distance.
Argument in the Negative.—In proof of this they adduce the fact that we are frequently mistaken in our estimate of the distance of objects. If there be more or fewer intervening objects than usual, if the atmosphere be more or less clear than usual, or any like circumstance affords a variation from our ordinary experience, we are misled as to the distance of the object. Hence we mistake the distance of ships at sea, or of objects on a prairie or a desert, the width of rivers, the height of steeples, towers, etc.
Further Argument in the Negative.—It is further contended that facts show that the impressions of sight alone, uncorrected by experience, do not convey the idea of distance at all, but that what we see seems to be in connection with the eye itself, until we learn the contrary by the aid of other senses. This, it is said, is the experience of persons who have been operated upon for cataract, particularly of a patient whose case is described by Cheselden, and who thought every thing which he saw, touched his eyes. It is said also to have been the same with Caspar Hauser, when first liberated from the long confinement of his dungeon, and permitted to look out upon the external world. The goodly landscape seemed to him to be a group of figures, drawn upon the window.
Force of this Argument.—This, however, is not inconsistent with the perception of externality by vision, since even what seems to be in contact with the eye, nay, what is known to be so, may still be known as external. Contact implies externality. It is very much to be doubted, moreover, whether the cases now referred to, coincide with the usual experience of those who are learning to see. The little child seems to recognize the externality and remoteness from his own person of the objects which attract his attention, as soon as he learns to observe surrounding objects at all, and, though he may not judge correctly of their relative distance from himself, never seems by his movements to suppose that they are in contact with his eye or with any part of his person. The young of animals, also, as soon as they are born, seem to perceive by the eye, the externality, the direction, and the distance of objects, and govern their movements accordingly. It is not, in these cases, a matter of experience, but of direct perception. These facts render it doubtful, to say the least, whether the common impression—that which in spite of all arguments to the contrary, is, and always will be made upon the mind in the act of vision, viz., that we see objects as external, as having locality, and as more or less remote from us—is not, after all, the correct impression.
Learning to judge of Distance not inconsistent with this View.—- Nor does it conflict with this view that we learn to judge of the true distance of objects, and are often deceived in regard to it. The measurement of distance, the more or less of it, is of course a matter of experience, a thing to be learned by practice. It does not follow, however, that we may not by the eye directly, and at first, perceive an object to be external, and removed from us, in other words distant, though we may not know at first how distant. The rays of light that come to us from this external object, may give us direct perception of the object as external, as extended, and as occupying apparently a given locality in space more or less remote, while at the same time it may be left to other senses and to experience to determine how great that distance is.
Questions as to Touch.—Passing now from the sense of sight to that of touch, we find similar questions discussed among philosophers respecting the precise information afforded by this sense. Does touch give us immediate perception of externality, extension, form, hardness, softness, etc., including the various mechanical properties of bodies? To this sense it has been common to ascribe these faculties of perception. They are so attributed by Reid, Upham, Wayland, and, I believe, by modern writers generally, with the exception of Brown and Hamilton.
Probability of another Source of Information.—It may be questioned, I think, whether, as regards some of these qualities at least, it is not rather the consciousness of resistance to muscular effort, than the sense of touch, properly speaking, that is the informing source. So, for example, as to hardness; the application of an external body lightly to the hand awakens the sense of touch, but conveys no idea of hardness. Let the same object be allowed to rest with gradually increasing weight upon the hand until it becomes painful, and we get the idea of weight, gravitation, but not of the hardness or impenetrability of the object. It is only when our muscular effort to move or penetrate the external body is met and resisted by the same, that we learn the impenetrability of the opposing body.
Other Perceptions attributable to the same Source.—So with regard to externality, extension, and form. When an external object, a cube, for example, or an ivory ball, is placed on the palm of the hand, sensation is awakened, but is that sensation necessarily accompanied with the perception of the external object as such? Does the mere tactual sensation, in the first instance, and of itself, inform us that there is something external to ourselves, that what we feel is not a part of our own organism? We are conscious of a change in the sensation of the part affected, but are we immediately conscious that this change is produced by something external? Let there be given, however, the consciousness of resistance to our muscular movements, as when the cube or ball, for instance, prevents the effort to close the hand, or when our locomotion is impeded by the presence of some obstacle, and will not the same resistance inform us of the extension of the resisting body, and so of its form and figure? We learn whereabout in space this resistance occurs, and where it ceases. The tactual sensation would indeed very soon come to our aid in this cognition, and serve as a guiding sense, even in the absence of the former. The question is, whether this alone would, in the first instance, give us such cognitions?
Our first Ideas of Extension, how derived.—We have had reference in this discussion only to the qualities of external bodies. There can be little question that our first ideas of extension are derived from our own sentient organism, the consciousness of sensations in different parts of the body, distinct from, and out of each other, thus affording the knowledge of an extended sentient organization. The idea of externality, or outness, and extension, thus acquired, the transition is easy from the perception of our own bodies as possessing these qualities, to the cognizance of the same qualities in external objects.
§ VI.—Credibility of our Sensations and Perceptions.
Denied by some.—There have always been those who were disposed to call in question the testimony of the senses. Such were the Eleatics and the Skeptics among the Greek philosophers, and there have not been wanting among the moderns minds of acuteness and ingenuity
