ON HUMAN UNDERSTANDING. 343 



exercise of the senses, which have been given to us for this purpose, and 

 which, to speak figuratively, may be regarded as the fingers of the mind in 

 feeling its way forward, and opening the shutters to the admission of that 

 pure and invigorating light, which in consequence breaks in upon it. 



It must be obvious, however, to every one who has attended to the opera- 

 tions of his senses, that there never is, nor can be, any direct communication 

 between the mind and the external objects the mind perceives, which are 

 usually, indeed, at some distance from the sense that gives notice of them. 

 Thus, in looking at a tree, it is the eye alone that really beholds the tree, 

 while the mind only receives a notice of its presence, by some means or 

 other, from the visual organ. So in touching this table, it is my hand alone 

 that comes in contact with it, and communicates to my mind a knowledge of 

 its hardness and other qualities. What, then, is the medium by which such 

 communication is maintained, which induces the mind, seated as it is in some 

 undeveloped part of the brain, to have a correspondent perception of the form, 

 size, colour, smell, and even distance of objects with the senses which are 

 seated on the surface of the body ; and which, at the same time that it con- 

 veys this information, produces such an additional effect, that the mind is able 

 at its option to revive the perception, or call up an exact notion or idea of 

 these qualities -at a distant period, or when the objects themselves are no 

 longer present? Is there, or is there not, any resemblance between the ex- 

 ternal or sensible object and the internal or mental idea or notion ? If there 

 be a resemblance, in what does that resemblance consist 1 and how is it pro- 

 duced and supported ? Does the external object throw off representative like- 

 nesses of itself in films, or under any other modification, so fine as to be able, 

 like the electric or magnetic aura, to pass without injury from the object to 

 the sentient organ, and from the sentient organ to the sensory ? Or has the 

 mind itself a faculty of producing, like a looking-glass, accurate countersigns, 

 intellectual pictures, or images, correspondent with the sensible images com- 

 municated from the external object to the sentient organ 1 If, on the con- 

 trary, there be no resemblance, are the mental perceptions mere notions or 

 intellectual symbols excited in it by the action of the external sense ; which, 

 while they bear no similitude to the qualities of the object discerned, answer 

 the purpose of those qualities, as letters answer the purpose of sounds ? Or 

 are we sure that there is any external world whatever 1 any thing beyond the 

 intellectual principle that perceives, and the sensations and notions that are 

 perceived ; or even any thing beyond those sensations and notions, those im- 

 pressions and ideas themselves ? 



Several of these questions may perhaps appear in no small degree whim- 

 sical and brain-sick, and more worthy of St. Luke's than of a scientific insti- 

 tution. But all of them, and perhaps as many more of a temperament as 

 wild as the wildest, have been asked, and insisted upon, and supported again 

 and again in different ages and countries, by philosophers of the clearest in- 

 tellects in other respects, and who had no idea of labouring under any such 

 mental infirmity, nor ever dreamed of the necessity of being blistered and 

 taking physic.* 



There is scarcely, however, an hypothesis which has been started in 

 modern times that cannot look for its prototype or suggestion among the 

 ancients; and it will hence be found most advantageous, and may perhaps 

 prove the shortest way to begin at the fountain-head, and to trace the different 

 currents which have flowed from it. That fountain-head is Greece, or at 

 least we may so regard it on the present occasion ; and the plan which I shall 

 request leave to pursue in the general inquiry before us will be, first of all, to 

 take a rapid sketch of the most celebrated speculations upon this subject to 

 which this well-spring of wisdom has given rise ; next, to foflow up the chief 

 ramifications which have issued from them in later periods ; and, lastly, to 

 summon, as by a quo warranto, the more prominent of those of our own day 

 to appear personally before the bar of this enlightened tribunal, for the pur- 



" See the author 1 ! Study of Medicine, vol. iv. p. 46. edit. 2. 1823 



