SECT. XVIII. 6. 7. O F S L E E P. 



This experiment is not eafy to be made at firft, 

 but by a few patient trials the fact appears very 

 certain ; and fhews clearly, that our ideas of ima- 

 gination are repetitions of the motions of the nerve, 

 which were originally occafioned by the ftimulus 

 of external bodies ; becaufe they equally expend 

 the icniorial power in the organ of fenfe. See Seel:. 

 HL 4. which is analogous to our being as much 

 fatigued by thinking as by labour. 



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

 waking reveries, 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 whilft I am thinking over the beau- 

 tiful 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 com- 

 mon attention to a favourite idea they do not hear 

 the voice of the companion, who accofts them, un- 

 lefs it is repeated with unufual energy. 



This perpetual miftake in dreams and reveries, 

 where our ideas of imagination are attended with a 

 belief of the prefence of external objeds, evinces 

 beyond a doubt, that all our ideas are repetitions of 

 the motions of the nerves of fenfe, by which they 

 were acquired ; and that this belief is not, as fome 

 late philofophers contend, an inttincl neceflarily 

 connected only with our perceptions. 



7. A curious queftion demands our attention in 

 this place ; as we do not diftinguifli in our dreams 

 and reveries between our perceptions of external 

 obje&s, and our ideas of them in their abfence, 

 how do we diftinguifli them at any time ? In a 

 dream, if the fweetnefs of fugar occurs to my ima- 

 gination, the whitenefs and hardnefs of it, which 



