Sect. XVIII. 6. OF SLEEP. , s7 



This experiment is not eaty to be made at firft, but by a few 

 patient trials the fact appears very certain ; and fhews clearly, 

 that our ideas of imagination are repetitions of the motions of 

 the nerve, which were originally occafioned by the flimulus of 

 external bodies ; becaufe they equally expend the fenforial power 

 in the organ of fenfe. See Seel:. III. 4. which is analogous to 

 pur being as much fatigued by thinking as by labour. 



6. Nor is it in our dreams alone, but even in our waking rev- 

 eries, and in great efforts of invention, fo great is the vivacity of our 

 ideas, that we do not for a time diftinguifh them from the real 

 prefence of fubftantial objects : though the external organs of 

 fenfe are open, and furrounded with their ufual ftimuli. Thus 

 whiHt I am thinking over the beautiful valley, through which I 

 yefterday travelled, I do not perceive the furniture of my room : 

 and there are fome, whofe waking imaginations are fo apt to run 

 into perfect reverie, that in their common attention to a favour- 

 ite idea they do*not hear the voice of the companion, who ac- 

 cofts them, unlefs it is repeated with unuiual energy. 



This perpetual miftake in dreams and reveries, where our 

 ideas of imagination are attended with a belief of the prefence 

 of external objects, evinces beyond a doubt, that all our ideas 

 are repetitions of the motions of the nerves of fenfe, by which 

 they were acquired 5 and that this belief is not, as fome late phi- 

 lofophers contend, an inftinct necelTarily connected only with 

 our perceptions. 



7. A curious queftion demands our attention in this place ; 

 as we do not diftinguifh in our dreams and reveries between our 

 perceptions of external objects, and our ideas of them in their 

 abfencc, how do we diftinguifh them at any time ? In a dream, 

 if the fweetnefs of fugar occurs to my imagination, the white- 

 nefs and hardnefs of it, which were ideas ufually connected with 

 the fweetnefs, immediately follow in the train ; and I believe a 

 material lump of fugar prefent before my fenfes : but in my 

 waking hours, if the fweetnefs occurs to my imagination, the 

 ftimulus of the table to my hand, or of the "window to my eye, 

 prevents the other ideas of the hardnefs and whitenefs of the fu- 

 gar from fucceeding ; and hence I perceive the fallacy, and dis- 

 believe the exiftence of objects correfpondent to thofe ideas, 

 whofe tribes or trains are broken by the ftimulus of other ob- 

 jects. And^jiurther in cur waking hours, we frequently exert 

 our volition in comparing prefent appearances with fuch, as we 

 have ufually obferved ; and thus correct the errors of one ien^ 

 by our general knowledge of nature by intuitive analogy. See 

 oect. XVII. 3.7. Whereas in dreams the power of volition is 

 fufpended, we can recollect and compare our prefcnt ideas with 



none 



