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WEmust，inthenextplace，investigatethesubjectofthedream，andfirstinquiretowhichofthefacultiesofthesoulitpresentsitself，i.e.whethertheaffectionisonewhichpertainstothefacultyofintelligenceortothatofsense—perception;forthesearetheonlyfacultieswithinusbywhichweacquireknowledge.
If，then，theexerciseofthefacultyofsightisactualseeing，thatoftheauditoryfaculty，hearing，and，ingeneralthatofthefacultyofsense—perception，perceiving;andiftherearesomeperceptionscommontothesenses，suchasfigure，magnitude，motion，&c.，whilethereareothers，ascolour，sound，taste，peculiar[eachtoitsownsense];andfurther，ifallcreatures，whentheeyesareclosedinsleep，areunabletosee，andtheanalogousstatementistrueoftheothersenses，sothatmanifestlyweperceivenothingwhenasleep;wemayconcludethatitisnotbysense—perceptionweperceiveadream.
Butneitherisitbyopinionthatwedoso.For[indreams]wenotonlyassert，e.g.thatsomeobjectapproachingisamanorahorse[whichwouldbeanexerciseofopinion]，butthattheobjectiswhiteorbeautiful，pointsonwhichopinionwithoutsense—perceptionassertsnothingeithertrulyorfalsely.Itis，however，afactthatthesoulmakessuchassertionsinsleep.Weseemtoseeequallywellthattheapproachingfigureisaman，andthatitiswhite.[Indreams]，too，wethinksomethingelse，overandabovethedreampresentation，justaswedoinwakingmomentswhenweperceivesomething;forweoftenalsoreasonaboutthatwhichweperceive.
So，too，insleepwesometimeshavethoughtsotherthanthemerephantasmsimmediatelybeforeourminds.Thiswouldbemanifesttoanyonewhoshouldattendandtry，immediatelyonarisingfromsleep，toremember[hisdreamingexperience].Therearecasesofpersonswhohaveseensuchdreams，those，forexample，whobelievethemselvestobementallyarrangingagivenlistofsubjectsaccordingtothemnemonicrule.Theyfrequentlyfindthemselvesengagedinsomethingelsebesidesthedream，viz.insettingaphantasmwhichtheyenvisageintoitsmnemonicposition.Henceitisplainthatnotevery\'phantasm\'insleepisameredream—image，andthatthefurtherthinkingwhichweperformthenisduetoanexerciseofthefacultyofopinion.
Somuchatleastisplainonallthesepoints，viz.thatthefacultybywhich，inwakinghours，wearesubjecttoillusionwhenaffectedbydisease，isidenticalwiththatwhichproducesillusoryeffectsinsleep.So，evenwhenpersonsareinexcellenthealth，andknowthefactsofthecaseperfectlywell，thesun，nevertheless，appearstothemtobeonlyafootwide.Now，whetherthepresentativefacultyofthesoulbeidenticalwith，ordifferentfrom，thefacultyofsense—perception，ineithercasetheillusiondoesnotoccurwithoutouractuallyseeingor[otherwise]perceivingsomething.Eventoseewronglyortohearwronglycanhappenonlytoonewhoseesorhearssomethingreal，thoughnotexactlywhathesupposes.Butwehaveassumedthatinsleeponeneithersees，norhears，norexercisesanysensewhatever.Perhapswemayregarditastruethatthedreamerseesnothing，yetasfalsethathisfacultyofsense—perceptionisunaffected，thefactbeingthatthesenseofseeingandtheothersensesmaypossiblybetheninacertainwayaffected，whileeachoftheseaffections，asdulyaswhenheisawake，givesitsimpulseinacertainmannertohis[primary]facultyofsense，thoughnotinpreciselythesamemanneraswhenheisawake.Sometimes，too，opinionsays[todreamers]justastothosewhoareawake，thattheobjectseenisanillusion;atothertimesitisinhibited，andbecomesamerefollowerofthephantasm.
Itisplainthereforethatthisaffection，whichwename\'dreaming\'，isnomereexerciseofopinionorintelligence，butyetisnotanaffectionofthefacultyofperceptioninthesimplesense.Ifitwerethelatteritwouldbepossible[whenasleep]tohearandseeinthesimplesense.
Howthen，andinwhatmanner，ittakesplace，iswhatwehavetoexamine.Letusassume，whatisindeedclearenough，thattheaffection[ofdreaming]pertainstosense—perceptionassurelyassleepitselfdoes.Forsleepdoesnotpertaintooneorganinanimalsanddreamingtoanother;bothpertaintothesameorgan.
Butsincewehave，inourworkOntheSoul，treatedofpresentation，andthefacultyofpresentationisidenticalwiththatofsense—perception，thoughtheessentialnotionofafacultyofpresentationisdifferentfromthatofafacultyofsense—perception;andsincepresentationisthemovementsetupbyasensoryfacultywhenactuallydischargingitsfunction，whileadreamappearstobeapresentation（forapresentationwhichoccursinsleep—whethersimplyorinsomeparticularway—iswhatwecalladream）：itmanifestlyfollowsthatdreamingisanactivityofthefacultyofsense—perception，butbelongstothisfacultyquapresentative.
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Wecanbestobtainascientificviewofthenatureofthedreamandthemannerinwhichitoriginatesbyregardingitinthelightofthecircumstancesattendingsleep.Theobjectsofsense—perceptioncorrespondingtoeachsensoryorganproducesense—perceptioninus，andtheaffectionduetotheiroperationispresentintheorgansofsensenotonlywhentheperceptionsareactualized，butevenwhentheyhavedeparted.
Whathappensinthesecasesmaybecomparedwithwhathappensinthecaseofprojectilesmovinginspace.Forinthecaseofthesethemovementcontinuesevenwhenthatwhichsetupthemovementisnolongerincontact[withthethingsthataremoved].Forthatwhichsettheminmotionmovesacertainportionofair，andthis，inturn，beingmovedexcitesmotioninanotherportion;andso，accordingly，itisinthiswaythat[thebodies]，whetherinairorinliquids，continuemoving，untiltheycometoastandstill.