It is indeed chiefly in consequence of this coalition of idea with impression that the operations of the mind and the business of human life are carried on with facility and despatch. If it were not for this law, every view of an external object would be attended with all the labour and protracted to all the length of a first examination. Experience would be in a great measure useless, and on every new examination of an object we should have to study it as if it were for the first time. But in consequence of this law, the slightest and shortest. impression on the senses is sufficient in all familiar cases. The least spark lights up the train of associated ideas. Perception becomes a language of which the chief use is to excite the correspondent series of thought, and the senses are seldom intensely and long employed but in the examination of new objects. The far greater part of what is supposed to be perception is only the body of ideas which a perception has awakened. If, from particular circumstances, our preconceptions or those accumulated antecedent ideas are uncommonly vivid, the slightest incident is sufficient to recall them, and every new impression that bears the remotest similitude to the original will revive the whole train of sensations: if, for example, a man come to an interview in very anxious expectation of a friend, he will sometimes for a moment mistake a mere stranger for the expected friend. After living a week in the centre of a deer park, I mistook the first flock of sheep I saw for deer. A peasant, whose mind is well stored with tales of ghosts sees a female figure clothed in white in a stone or a cow. In these cases the previous ideas modify the perception so as to produce mistake, usually referred to the senses, but which is really referable to the mind. In the approaches, and still more under the influence, of insanity, an idea may predominate so strongly as to assimilate to itself every perception to which it bears the most distant resemblance. In Irwin's Voyage on the Red Sea,' we read of a young man whose mind was so constantly haunted by the dread of assassination by the Arabs, that, looking one day earnestly at the bottom of the boat, he exclaimed, 'The darts of the Arabs,' nor could he be

6

404

HOMOGENEOUSNESS OF PERCEPTION AND IDEA.

convinced that what he saw were merely reeds. Such was the first indication that he gave of mental derangement.

These observations will be sufficient to prove the homogeneousness of perceptions and ideas, and their capacity of being thoroughly blended so as to form one whole. The principle is implied, though not unfolded, in Berkeley's own "Theory of Vision,' for it supposes the ideas of objects derived from touch to be excited by the perceptions which enter through the eye, and the idea to be so constantly associated with the perception that they never can be separated. But its importance deserves that it should be distinctly considered as one of the principal of the secondary laws of thought; and that importance will appear to be still greater if I am successful in deducing from it a new and more probable theory of the acquired perceptions of sight.

It has already been said that superficial distance (or space considered merely in length and breadth) is an original object of vision. It must indeed be as much so as colour, since it is manifestly inconceivable that we should see unextended colour. Figure is bounded extension; and these three perceptions-namely, colour, superficial extension, and superficial figure-are the three coeval and inseparable perceptions of sight which must have entered the mind together on the first exercise of the faculty of vision, and which can never be imagined to exist separate from each other. The generally received doctrine, that distance is not an original object of sight, is ambiguously expressed. As superficial space is an original object of sight, so must the distance between two points which (to borrow an expression from subsequent experience) are in the same plane; otherwise one circle would not originally appear larger than another. That which is not an original object of vision is distance from the eye or outness, and the manner in which we acquire this notion is the object of the present enquiry.

A child has at first no conception that any part of the picture presented to his eyes is composed of his own figure. He views his hand, body, or foot with the same interest as the trees, stones, &c. He has no idea of sentience connected with one object more than another, nor a thought, like what

CORRECTION OF IMPRESSION.

405

he afterwards acquires, that he is himself present in one part of the picture, from which the distances of the rest are measured.

The notion of his person is acquired by observing that sensation is always connected with certain parts of the picture, and that those parts never vary like the others in distinctness, size, colour, &c. His own figure is then made up of a certain observed portion of the picture, which is a constant, uniform, unvarying object in every different picture of objects which are unceasingly changing their aspects.

Let us now suppose him to look at his finger, held in that position in which all the parts of it are at nearly an equal distance from the eye. He repeats the observation so often that he acquires a full notion of the superficial distance of all the parts of the finger from each other. Suppose the finger then to be placed somewhat obliquely, the more distant parts of its surface will make a smaller impression on the eye (that is, will subtend a less angle) than they did before. But the idea of these more distant parts gained from former observation will be immediately excited. This idea will correct the impression made on the sense, and thus the more distant parts will seem to be as large as before the finger was moved into an oblique position. When the child has looked often enough at all the parts of his finger, a glimpse of one part of its surface will excite the ideas of all the other parts of it. After a thousand views of the finger in all directions, he never looks at one side without synchronous ideas of the other side: it is hardly observable where impression ends and idea begins. He cannot see the knuckles in a fore-shortened view without synchronous ideas of the parts interjacent (for they are like the further side of the finger in the preceding case, parts now unseen, but of which there are familiar ideas in the mind), and he cannot have these ideas of the interjacent parts without imagining the knuckles at something like their real distance from each other; this gives outness or distance from the eye, which differs from superficial distance only in this respect, that the eye must be considered as one of the points between which space is extended.

406

VISUAL PERCEPTIONS AND IDEAS.

A child believes his finger in all views to be the same object, because he never has a new impression of it without some recognised part of a former impression along with it, and also some idea of parts obscure and unseen.

If the more simple parts of this process are distinctly apprehended, there will be no difficulty in conceiving those which are more complicated.

I look at a globe; no more of it than one hemisphere can be the direct object of vision. But I have no separate notion of the hemisphere I have before seen as soon as I discern the one which I now see. The idea of the invisible part of the globe instantaneously blends with the perception of that which is visible, and they jointly form my notion of the globe.

There is a certain distance from the eye at which an object must be placed in order that it may be most distinctly seen. This is the nearest distance at which the eye can distinctly take in the whole object: when the object is brought nearer the eye sees only a part of it; when it is removed to a greater distance the impression on the sense is smaller. This is the largest possible view of the object, and that which leaves behind the fullest and clearest idea of it. This perception is, in other respects, of such importance that the mind naturally recurs to it more frequently than to any other. The idea, then, of the object seen at this distance is in itself the most full and distinct, and is associated with the greatest number of other ideas, as well as with the strongest emotions.

Here, then, is a visual idea of an object which may be substituted for the tangible magnitude of Berkeley. This idea furnishes what we call the real magnitude of the object. All the other perceptions of the object, being comparatively indistinct and uninteresting, are chiefly useful in calling up this idea. Thus a standard visual idea of every object is formed, which instantly blends with every fugitive perception and corrects it. A visual perception is a sign which excites the standard visual idea, and the whole of that process is performed by the sense of sight alone, for which Berkeley called in the assistance of the sense of touch.

INDEX.