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their continuance in the mind is not regulated by us, we ascribe to the objects of imagination an independent and permanent existence, as we do when awake to the objects of perception. The same thing happens in those kinds of madness, in which a particular idea takes possession of the attention, and occupies it to the exclusion of every thing else. Indeed, madness seems in many cases to arise entirely from a suspension of the influence of the will over the succession of our thoughts; in consequence of which, the objects of imagination appear to have an existence independent of you our volition ; and are therefore, agreeably to the foregoing doctrine, mistaken for realities.

Numberless other illustrations of the same general fact occur to me; but the following is, I think, one of the most striking. I mention it, in preference to the rest, as it appears to me to connect the doctrine in question with some principles which are now universally admitted among philosophers.

The distinction between the original and the acquired perceptions of sight, is familiarly known to every one who has the slightest acquaintance with the elements of optics. That this sense, prior to experience, conveys to us the notion of extension in two dimensions only, and that it gives us no information concerning the distances at which objects are placed from the eye, are propositions which nobody, I presume, in the present state of science, will be disposed to controvert. In what manner we are enabled, by a comparison between the perceptions of sight and those of touch, to extend the province of the former sense to a variety of qualities originally perceived by the latter sense only,

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optical writers have explained at great length ; but it is not necessary for my present purpose to enter into any particular details with respect to their reasonings on the subject. It is sufficient for me to remark, that, according to the received doctrine, the original per: ceptions of sight become, in consequence of experience, signs of the tangible qualities of external objects, and of the distances at which they are placed from the organ; and that, although the knowledge we obtain, in this manner, of these qualities and distances, feems, from early and constant habits, to be an instantaneous perception ; yet, in many cases, it implies an exercise of the judgment, being founded on a comparison of a variety of different circumstances.

From these principles, it is an obvious consequence, that the knowledge we obtain, by the eye, of the tangible qualities of bodies, involves the exercise of conception, according to the definition of that power which has already been given. In ordinary discourse, indeed, we ascribe this knowledge, on account of the instantaneousness with which it is obtained, to the power of perception ; but if the common doctrine on the subject be just, it is the result of a complex operation of the mind; comprehending, first, the perception of those qualities, which are the proper and original objects of fight; and, secondly, the conception of those tangible qualities of which the original perceptions of sight are found from experience to be the figns. The notions, therefore, we form, by means of the eye, of the tangible qualities of bodies, and of the distances of these objects from the organ, are mere conceptions ; strongly, and indeed indissolubly, affociated, by early and constant habit, with the original perceptions of fight.

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When we open our eyes on a magnificent prospect, the various distances at which all its different parts are placed from the eye, and the immense extent of the whole scene before us, seem to be perceived as imme. diately, and as instantaneously, by the mind, as the coloured surface which is painted on the retina. The truth, however, unquestionably is, that this variety of distance, and this immensity of extent, are not objects of sense but of conception ; and the notions we form of them when our eyes are open, differ from those we should form of them with our eyes shut, only in this, that they are kept steadily in the view of the mind, by being strongly associated with the senfations of colour, and with the original perceptions of sight. -This observation will be the more readily admitted, if it be considered, that, by a skilful imitation of a natural landscape, in a common fhew-box, the mind may be led to form the same notions of variety of diftance, and even of immense extent, as if the original scene were presented to our senses : and that, ale though, in this case, we have a speculative conviction that the sphere of our vision only extends to a few inches ; yet so strong is the association between the original perceptions of fight, and the conceptions which they habitually produce, that it is not possible for us, by any effort of our will, to prevent these conceptions from taking place.

From these observations it appears, that when the conceptions of the mind are rendered steady and permanent, by being strongly associated with any sensible

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impression, they command our belief no less than our actual perceptions; and, therefore, if it were possible for us, with our eyes shut, to keep up, for a length of time, the conception of any sensible object, we should, as long as this effort continued, believe that the object was present to our senses. It

appears to me to be no slight confirmation of these remarks, that although, in the dark, the illusions of imagination are much more liable to be mistaken for realities, than when their momentary effects on the belief are continually checked and corrected by the objects which the light of day presents to our perceptions; yet, even total darkness is not so alarming to a person impressed with the vulgar stories of apparitions, as a faint and doubtful twilight, which affords to the conceptions an opportunity of fixing and prolonging their existence, by attaching themselves to something which is obscurely exhibited to the eye.--In like manner, when we look through a fog, we are frequently apt to mistake a crow for a man; and the conception we have, upon such an occasion, of the human figure, is much more distinct and much more steady, than it would be possible for us to form, if we had no sensible obje& before us; insomuch that when, on a more attentive observation, the crow shrinks to its own dimensions, we find it impossible, by any effort, to conjure up the phantom which a moment before we seemed to perceive.

If these observations are admitted, the effects which exhibitions of fiétitious distress produce on the mind, will

appear less wonderful, than they are supposed to be. During the representation of a tragedy, I ac

knowledge, knowledge, that we have a general conviction that the whole is a fiction ; but, I believe, it will be found, that the violent emotions which are sometimes produced by the distresses of the stage, take their rise, in most cases, from a momentary belief, that the diftresses are real. I say, in most cases; because, I acknowledge, that independently of any such belief, there is something contagious in a faithful expression of any of the passions.

The emotions produced by tragedy are, upon this supposition, somewhat analogous to the dread we feel when we look down from the battlement of a tower. In both cases, we have a general conviction, that there is no ground for the feelings we experience; but the momentary influences of imagination are lo powerful as to produce these feelings, before reflexion has time to come to our relief.

* With respect to the dread which we feel in looking down from the batılement of a tower, it is curious to remark the effects of habit in gradually destroying it. The manner in which habit operates in this case, seems to be by giving us a command over our thoughts, so as to enable us to withdraw our attention from the precipice before us, and direct it to any other object at pleasure. It is thus that the mason and the sailor not only can take precautions for their own safety, but remain completely masters of them. selves in fituations where other men, engrossed with their imaginary danger, would experience a total suspension of their faculties. Any strong passion which occupies the mind produces, for the mo. ment, the same effect with habit. A person alarmed with the apprehenfion of fire, has been known to escape from the top of a house, by a path which, at another time, he would have considered as impracticable ; and soldiers, in mounting a breach, are said to have

fometimes found their way to the enemy, by a route which apa peared inaccessible after their violent passions had subsided.

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