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

Numberless other illuftrations of the fame general fact occur to me; but the following is, I think, one of the most striking. I mention it, in preference to the reft, as it appears to me to connect the doctrine in question with fome principles which are now univerfally admitted among philofophers.

The distinction between the original and the acquired perceptions of fight, is familiarly known to every one who has the flightest acquaintance with the elements of optics. That this sense, prior to experience, conveys to us the notion of extenfion in two dimensions only, and that it gives us no information concerning the distances at which objects are placed from the eye, are propofitions which nobody, I prefume, in the prefent ftate of science, will be difpofed to controvert. In what manner we are enabled, by a comparison between the perceptions of fight and thofe of touch, to extend the province of the former sense to a variety of qualities originally perceived by the latter fenfe only, optical

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optical writers have explained at great length; but it is not neceffary for my present purpose to enter into any particular details with respect to their reafonings on the fubject. It is fufficient for me to remark, that, according to the received doctrine, the original perceptions of fight become, in confequence of experience, figns 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 thefe qualities and diftances, feems, from early and conftant habits, to be an inftantaneous perception; yet, in many cafes, 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 confequence, that the knowledge we obtain, by the eye, of the tangible qualities of bodies, involves the exercife of conception, according to the definition of that power which has already been given. In ordinary discourse, indeed, we afcribe this knowledge, on account of the instantaneousness with which it is obtained, to the power of perception; but if the common doctrine on the fubject be juft, it is the refult of a complex operation of the mind; comprehending, first, the perception of thofe qualities, which are the proper and original objects of fight; and, fecondly, the conception of those tangible qualities of which the original perceptions of fight 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 diftances of these objects from the organ, are mere conceptions; strongly, and indeed indiffolubly, affo

ciated,

ciated, by early and constant habit, with the original perceptions of fight.

When we open our eyes on a magnificent profpect, the various distances at which all its different parts are placed from the eye, and the immenfe extent of the whole scene before us, feem to be perceived as imme. diately, and as inftantaneously, by the mind, as the coloured furface which is painted on the retina. The truth, however, unquestionably is, that this variety of distance, and this immenfity 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 fhut, only in this, that they are kept fteadily in the view of the mind, by being ftrongly affociated with the fenfations of colour, and with the original perceptions of fight. -This obfervation will be the more readily admitted, if it be confidered, that, by a skilful imitation of a natural landscape, in a common fhew-box, the mind may be led to form the fame notions of variety of diftance, and even of immenfe extent, as if the original scene were prefented to our fenfes and that, although, in this cafe, we have a fpeculative conviction that the sphere of our vifion only cxtends to a few inches; yet fo ftrong is the affociation between the original perceptions of fight, and the conceptions which they habitually produce, that it is not poffible 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 affociated with any fenfible

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impreffion,

impreffion, they command our belief no lefs than our actual perceptions; and, therefore, if it were poffible for us, with our eyes fhut, to keep up, for a length of time, the conception of any sensible object, we fhould, as long as this effort continued, believe that the object was present to our senses.

It appears to me to be no flight confirmation of these remarks, that although, in the dark, the illufions 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 fo alarming to a perfon impreffed with the vulgar ftories of apparitions, as a faint and doubtful twilight, which affords to the conceptions an opportunity of fixing and prolonging their existence, by attaching themfelves to fomething which is obfcurely 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 fuch an occasion, of the human figure, is much more diftinct and much more steady, than it would be poffible for us to form, if we had no fenfible object before us; infomuch that when, on a more attentive obfervation, the crow fhrinks to its own dimenfions, we find it impoffible, by any effort, to conjure up the phantom which a moment before we feemed to perceive.

If these obfervations are admitted, the effects which exhibitions of fictitious diftrefs produce on the mind, will appear lefs wonderful, than they are supposed to be. During the representation of a tragedy, I acknowledge,

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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 diftreffes of the stage, take their rife, in most cases, from a momentary belief, that the diftreffes are real. I fay, in most cases; because, I acknowledge, that independently of any fuch belief, there is fomething contagious in a faithful expreffion of any of the paffions.

The emotions produced by tragedy are, upon this fuppofition, fomewhat analogous to the dread we feel when we look down from the battlement of a tower. In both cafes, we have a general conviction, that there is no ground for the feelings we experience; but the momentary influences of imagination are so 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 battlement of a tower, it is curious to remark the effects of habit in gradually deftroying it. The manner in which habit. operates in this case, seems to be by giving us a command over our thoughts, fo 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 failor not only can take precautions for their own fafety, but remain completely masters of themfelves in fituations where other men, engroffed with their imaginary danger, would experience a total fufpenfion of their faculties. Any ftrong paffion which occupies the mind produces, for the moment, the same effect with habit. A perfon 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 confidered as impracticable; and foldiers, in mounting a breach, are faid to have fometimes found their way to the enemy, by a route which appeared inacceffible after their violent paffions had fubfided.

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