384 



ON HUMAN UNDERSTANDING. 



knowledge of its hardness and other quahties. What then is the medmin 

 by which such communication is maintained which induces the mind^ 

 seated as i^is in some undeveloped part of the brain, to have a corres- 

 pondent perception of the form, size, colout, 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 conveys this information, produces such an 

 additional effect that the mind is able at its option to revive the percep- 

 tion, 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 external or sensible Object 

 and the internal or mental idea or notion ? If there be a resemblance, in 

 what does that resemblance consist ? and how is it produced and sup- 

 ported ? Does the external object throw off representative likenesses 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 the faculty of producing, like a looking-glass, accurate coun- 

 ter-signs, intellectual pictures or images, correspondent with the sensible 

 images communicated from the external object to the sentient organ ? If, 

 on the contrary, 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 simiHtude 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 ? 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 impressions and ideas themselves ? 



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

 sical and brain-sick, and more worthy of Saint Luke's than of a scientific 

 institution. 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 

 a^ain and again in different ages and countries, by philosophers of the 

 clearest intellects in other respects, and who had no idea of labouring under 

 any such mental infirmity, nor ever dreamed of the necessity of being blis- 

 tered and taking physic* 



There is scarcely, however, a 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 dif- 

 ferent 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 follow 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 enlight- 

 ened tribunal, for the purpose of trying their comparative pretensions, and 

 of submitting them to your impartial award. 



The principal systems that were started among the philosophers of 

 Greece to explain the origin and value of human knowledge were those of 



* See the Author's Study of MedicinCj vol. iv. p. 46. Edit, 2. 1825- 



