Sight is of all the senses the most communicative as a vehicle of information, and consequently the one in which there is the least immediate consciousness of pleasure or pain in the exercise. Most of the knowledge, however, which this sense, in its matured state, conveys to us, belongs to its acquired, not to its original power, and is the result, not of a direct perception, but of an inference from a perception.
In sight, as in the other senses, the direct perception is produced by contact; and the proper object of this sense is not the distant body from which the rays of light are emitted or reflected, but the reflection of the organ of sight (the retina and the nervous system connected with it) produced by the rays impinging upon it.
The essential characteristics of this affection are brightness and colour, which, however, are necessarily accompanied by a consciousness of extension; or a luminous point, however small, must itself occupy some portion of space, and can be perceived only as in contrast to a surrounding expanse of obscure or differently-coloured surface.
The immediate object of sight being in contact with the extremities of the optic nerve of the person seeing, it is as impossible for two persons simultaneously to see the same object with their eyes as to touch the same spot with their fingers; and every movement of the eye which brings a different portion of rays into contact with the organ, produces a different object of vision.
The object of vision being thus neither the rays alone nor the organ alone, but the organ as affected by the rays; and the sensation of colour being a purely organic affection; it follows that sight, like the other senses, gives us no immediate knowledge of an extra-organic world; though it is immediately cognisant of extension, and therefore of matter, as presented in the organism itself.
Hence we have no immediate perception by sight of the figure, the size, or the distance of bodies; and we cannot, in strict accuracy, be said to see a distant body, such as the sun, at all; though in practice the direct perception becomes so intimately united with the indirect inference, that it is difficult to imagine that either can exist apart from the other.
Admitting this view of the true object of sight, which may be regarded as established by physiological as well as psychological testimony, we may notice some remarkable contrasts between the presented object, or that which we actually see, and the represented object, or that which we appear to see. The presented object is on the surface of the retina: the represented object appears without, and at a greater or less distance from the eye.
The presented object is of such a size as can be contained within the spectator’s visual organism: the represented object may be many times larger than his whole body. The presented object is a flat surface: the represented object is a solid body. The presented object is inverted: the represented object is erect.
The presented object is double, there being a distinct image on the retina of each eye: the represented object is generally single, the two images being in normal vision united into one body. These and other apparent anomalies in the exercise of the senses will be discussed under the head of “ Acquired Perceptions.”
***Excerpt from Henry Longueville Mansel, B.d.. Metaphysics or the philosophy of consciousness