optical writers have explained at great length; but it is not necessary for my prefent purpose to enter into any particular details with refpect to their reafonings on the subject. It is fufficient for me to remark, that, according to the received doctrine, the original perceptions of fight become, in confequence of experience, figus 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 instantaneous perception; yet, in many cafes, it implies an exercise of the judgment, being founded on a comparison of a variety of different circumftances.

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 difcourse, indeed, we afcribe this knowledge, on account of the inftantaneousness 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, firft, the perception of thofe qualities, which are the proper and original objects of fight; and, fecondly, the conception of thofe 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 thefe objects from the organ, are mere conceptions; ftrongly, and indeed indiffolubly, affociated,

ciated, by early and conftant 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 immediately, 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 immensity of extent, are not objects of fenfe 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 observation 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 dif. tance, and even of immenfe extent, as if the original fcene were prefented to our fenfes and that, although, in this cafe, we have a fpeculative conviction that the sphere of our vifion only extends 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 thefe obfervations it appears, that when the conceptions of the mind are rendered steady and permanent, by being ftrongly affociated with

L3

any fenfible

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 fenfible object, we fhould, as long as this effort continued, believe that the object was prefent to our fenfes.

It appears to me to be no flight confirmation of thefe remarks, that although, in the dark, the illufions of imagination are much more liable to be miftaken for realities, than when their momentary effects on the belief are continually checked and corrected by the objects which the light of day prefents 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 exiftence, 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 miftake a crow for a man; and the conception we have, upon fuch an occafion, of the human figure, is much more diftin&t and much more steady, than it would be poflible 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 impoflible, by any effort, to conjure up the phantom which a moment before we seemed to perceive.

If thefe obfervations are admitted, the effects which exhibitions of fictitious diftrefs produce on the mind, will appear lefs wonderful, than they are fuppofed to be. During the reprefentation 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 diftreffes of the ftage, take their rife, in most cafes, from a momentary belief, that the diftreffes are real. I fay, in moft cafes; becaufe, I acknowledge, that independently of any fuch belief, there is fomething contagious in a faithful expreffion of the paffions.

of

any

The emotions produced by tragedy are, upon this fuppofition, somewhat 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 thefe 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 mafon and the failor not only can take precautions for their own safety, 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 fame 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 ap peared inacceffible after their violent paffions had fubfided.

CHAPTER FOURTH.

Of Abstraction.

SECTION I.

General Obfervations on this Faculty of the Mind.

THE origin of appellatives, or, in other words, the origin of thofe claffes of objects which, in the schools, are called genera, and fpecies, has been confidered by fome philofophers as one of the moft dif ficult problems in metaphyfics. The account of it which is given by Mr. Smith, in his Differtation on the Origin of Languages, appears to me to be equally fimple and fatisfactory.

"The affignation" (fays he) "of particular names, "to denote particular objects; that is, the inftitu❝tion of nouns fubftantive; would probably be one "of the first steps towards the formation of Language. "The particular cave, whofe covering fheltered the "favage from the weather; the particular tree, whofe "fruit relieved his hunger; the particular fountain, "whose water allayed his thirst; would first be de "nominated by the words, cave, tree, fountain; or "by whatever other appellations he might think

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proper, in that primitive jargon, to mark them. Afterwards, when the more enlarged experience of