122 PRODUCTION OF IDEAS. SECT. XIV. 2. 



the fenfe of hearing resembles the vibrations of ex- 

 ternal bodies in its capability of being ftimulated 

 into motion by thofe vibrations ; and though our 

 other organs of fenfe refemble the bodies, that fti- 

 mulate them, in their capability of being ftimulat- 

 ed by them ; and we hence become acquainted with 

 thefe properties of the external world ; yet as we 

 can repeat all thefe motions of our organs of fenfe 

 by the efforts of volition, or in confequence of 

 the fenfation of pleafure or pain, or by their affo- 

 ciation with other fibrous motions, as happens in 

 our reveries or in fleep, there would ftill appear to 

 be fome difficulty in demonftrating the exiftence 

 of any thing external to us. 



In our dreams we cannot determine this circum- 

 fiance, becaufe our power of volition is fufpended, 

 and the {Hmuli of external objeclss are excluded ; 

 but in our waking hours we can compare our ideas 

 belonging to one fenfe with thofe belonging to 

 another, and can thus diftinguifh the ideas occa- 

 fioned by irritation from thofe excited by fenfation, 

 volition, or affociation. Thus if the idea of the 

 fweetnefs of fugar fhould be excited in our dreams, 

 the whitenefs and hardnefs of it occur at the fame 

 time by affociation ; and we believe a material 

 lump of fugar prefent before us. But if, in our 

 waking hours, the idea of the fweetnefs of fugar 

 occurs to us, the ftimuli of furrounding objects, 

 as the edge of the table, on which we prefs, or 

 green colour of the grafs, on which we tread, pre- 

 vent the other ideas of the hardnefs and whitenefs 

 of the fugar from being exerted by affociation. 

 Or if they fhould occur, we voluntarily compare 

 them with the irritative ideas of the table or grafs 

 above mentioned, and detect their fallacy. We 

 can thus diftinguifh the ideas caufed by the flimuli 

 of external objeds from thofe, which are intro- 

 duced by affociation, fenfation, or volition; and 



during 



