ButtherearemanyothercircumstanceswhichevincethatwhatIhaveallegedisthetruecauseofthemotionoftheblood：thus，inthefirstplace，thedifferencethatisobservedbetweenthebloodwhichflowsfromtheveins，andthatfromthearteries，canonlyarisefromthis，thatbeingrarefied，and，asitwere，distilledbypassingthroughtheheart，itisthinner，andmorevivid，andwarmerimmediatelyafterleavingtheheart，inotherwords，wheninthearteries，thanitwasashorttimebeforepassingintoeither，inotherwords，whenitwasintheveins；andifattentionbegiven，itwillbefoundthatthisdifferenceisverymarkedonlyintheneighborhoodoftheheart；andisnotsoevidentinpartsmoreremotefromit。Inthenextplace，theconsistencyofthecoatsofwhichthearterialveinandthegreatarteryarecomposed，sufficientlyshowsthatthebloodisimpelledagainstthemwithmoreforcethanagainsttheveins。Andwhyshouldtheleftcavityoftheheartandthegreatarterybewiderandlargerthantherightcavityandthearterialvein，wereitnotthatthebloodofthevenousartery，havingonlybeeninthelungsafterithaspassedthroughtheheart，isthinner，andrarefiesmorereadily，andinahigherdegree，thanthebloodwhichproceedsimmediatelyfromthehollowvein？Andwhatcanphysiciansconjecturefromfeelingthepulseunlesstheyknowthataccordingasthebloodchangesitsnatureitcanberarefiedbythewarmthoftheheart，inahigherorlowerdegree，andmoreorlessquicklythanbefore？Andifitbeinquiredhowthisheatiscommunicatedtotheothermembers，mustitnotbeadmittedthatthisiseffectedbymeansoftheblood，which，passingthroughtheheart，isthereheatedanew，andthencediffusedoverallthebody？Whenceithappens，thatifthebloodbewithdrawnfromanypart，theheatislikewisewithdrawnbythesamemeans；andalthoughtheheartwereas—hotasglowingiron，itwouldnotbecapableofwarmingthefeetandhandsasatpresent，unlessitcontinuallysentthithernewblood。Welikewiseperceivefromthis，thatthetrueuseofrespirationistobringsufficientfreshairintothelungs，tocausethebloodwhichflowsintothemfromtherightventricleoftheheart，whereithasbeenrarefiedand，asitwere，changedintovapors，tobecomethick，andtoconvertitanewintoblood，beforeitflowsintotheleftcavity，withoutwhichprocessitwouldbeunfitforthenourishmentofthefirethatisthere。Thisreceivesconfirmationfromthecircumstance，thatitisobservedofanimalsdestituteoflungsthattheyhavealsobutonecavityintheheart，andthatinchildrenwhocannotusethemwhileinthewomb，thereisaholethroughwhichthebloodflowsfromthehollowveinintotheleftcavityoftheheart，andatubethroughwhichitpassesfromthearterialveinintothegrandarterywithoutpassingthroughthelung。Inthenextplace，howcoulddigestionbecarriedoninthestomachunlesstheheartcommunicatedheattoitthroughthearteries，andalongwiththiscertainofthemorefluidpartsoftheblood，whichassistinthedissolutionofthefoodthathasbeentakenin？Isnotalsotheoperationwhichconvertsthejuiceoffoodintobloodeasilycomprehended，whenitisconsideredthatitisdistilledbypassingandrepassingthroughtheheartperhapsmorethanoneortwohundredtimesinaday？Andwhatmoreneedbeadducedtoexplainnutrition，andtheproductionofthedifferenthumorsofthebody，beyondsaying，thattheforcewithwhichtheblood，inbeingrarefied，passesfromthehearttowardstheextremitiesofthearteries，causescertainofitspartstoremaininthemembersatwhichtheyarrive，andthereoccupytheplaceofsomeothersexpelledbythem；
andthataccordingtothesituation，shape，orsmallnessoftheporeswithwhichtheymeet，someratherthanothersflowintocertainparts，inthesamewaythatsomesievesareobservedtoact，which，bybeingvariouslyperforated，servetoseparatedifferentspeciesofgrain？And，inthelastplace，whataboveallishereworthyofobservation，isthegenerationoftheanimalspirits，whicharelikeaverysubtlewind，orratheraverypureandvividflamewhich，continuallyascendingingreatabundancefromthehearttothebrain，thencepenetratesthroughthenervesintothemuscles，andgivesmotiontoallthemembers；sothattoaccountforotherpartsofthebloodwhich，asmostagitatedandpenetrating，arethefittesttocomposethesespirits，proceedingtowardsthebrain，itisnotnecessarytosupposeanyothercause，thansimply，thatthearterieswhichcarrythemthitherproceedfromtheheartinthemostdirectlines，andthat，accordingtotherulesofmechanicswhicharethesamewiththoseofnature，whenmanyobjectstendatoncetothesamepointwherethereisnotsufficientroomforall（asisthecasewiththepartsofthebloodwhichflowforthfromtheleftcavityoftheheartandtendtowardsthebrain），theweakerandlessagitatedpartsmustnecessarilybedrivenasidefromthatpointbythestrongerwhichaloneinthiswayreachitI
hadexpoundedallthesematterswithsufficientminutenessinthetreatisewhichIformerlythoughtofpublishing。Andafterthese，Ihadshownwhatmustbethefabricofthenervesandmusclesofthehumanbodytogivetheanimalspiritscontainedinitthepowertomovethemembers，aswhenweseeheadsshortlyaftertheyhavebeenstruckoffstillmoveandbitetheearth，althoughnolongeranimated；whatchangesmusttakeplaceinthebraintoproducewaking，sleep，anddreams；howlight，sounds，odors，tastes，heat，andalltheotherqualitiesofexternalobjectsimpressitwithdifferentideasbymeansofthesenses；howhunger，thirst，andtheotherinternalaffectionscanlikewiseimpressuponitdiversideas；whatmustbeunderstoodbythecommonsense（sensuscommunis）inwhichtheseideasarereceived，bythememorywhichretainsthem，bythefantasywhichcanchangetheminvariousways，andoutofthemcomposenewideas，andwhich，bythesamemeans，distributingtheanimalspiritsthroughthemuscles，cancausethemembersofsuchabodytomoveinasmanydifferentways，andinamannerassuited，whethertotheobjectsthatarepresentedtoitssensesortoitsinternalaffections，ascantakeplaceinourowncaseapartfromtheguidanceofthewill。Norwillthisappearatallstrangetothosewhoareacquaintedwiththevarietyofmovementsperformedbythedifferentautomata，ormovingmachinesfabricatedbyhumanindustry，andthatwithhelpofbutfewpiecescomparedwiththegreatmultitudeofbones，muscles，nerves，arteries，veins，andotherpartsthatarefoundinthebodyofeachanimal。SuchpersonswilllookuponthisbodyasamachinemadebythehandsofGod，whichisincomparablybetterarranged，andadequatetomovementsmoreadmirablethanisanymachineofhumaninvention。AndhereIspeciallystayedtoshowthat，weretheresuchmachinesexactlyresemblingorgansandoutwardformanapeoranyotherirrationalanimal，wecouldhavenomeansofknowingthattheywereinanyrespectofadifferentnaturefromtheseanimals；butifthereweremachinesbearingtheimageofourbodies，andcapableofimitatingouractionsasfarasitismorallypossible，therewouldstillremaintwomostcertaintestswherebytoknowthattheywerenotthereforereallymen。Ofthesethefirstisthattheycouldneverusewordsorothersignsarrangedinsuchamannerasiscompetenttousinordertodeclareourthoughtstoothers：forwemayeasilyconceiveamachinetobesoconstructedthatitemitsvocables，andeventhatitemitssomecorrespondenttotheactionuponitofexternalobjectswhichcauseachangeinitsorgans；forexample，iftouchedinaparticularplaceitmaydemandwhatwewishtosaytoit；ifinanotheritmaycryoutthatitishurt，andsuchlike；butnotthatitshouldarrangethemvariouslysoasappositelytoreplytowhatissaidinitspresence，asmenofthelowestgradeofintellectcando。Thesecondtestis，thatalthoughsuchmachinesmightexecutemanythingswithequalorperhapsgreaterperfectionthananyofus，theywould，withoutdoubt，failincertainothersfromwhichitcouldbediscoveredthattheydidnotactfromknowledge，butsolelyfromthedispositionoftheirorgans：forwhilereasonisanuniversalinstrumentthatisalikeavailableoneveryoccasion，theseorgans，onthecontrary，needaparticulararrangementforeachparticularaction；whenceitmustbemorallyimpossiblethatthereshouldexistinanymachineadiversityoforganssufficienttoenableittoactinalltheoccurrencesoflife，inthewayinwhichourreasonenablesustoact。Again，bymeansofthesetwotestswemaylikewiseknowthedifferencebetweenmenandbrutes。Foritishighlydeservingofremark，thattherearenomensodullandstupid，notevenidiots，astobeincapableofjoiningtogetherdifferentwords，andtherebyconstructingadeclarationbywhichtomaketheirthoughtsunderstood；andthatontheotherhand，thereisnootheranimal，howeverperfectorhappilycircumstanced，whichcandothelike。Nordoesthisinabilityarisefromwantoforgans：forweobservethatmagpiesandparrotscanutterwordslikeourselves，andareyetunabletospeakaswedo，thatis，soastoshowthattheyunderstandwhattheysay；inplaceofwhichmenborndeafanddumb，andthusnotless，butrathermorethanthebrutes，destituteoftheorganswhichothersuseinspeaking，areinthehabitofspontaneouslyinventingcertainsignsbywhichtheydiscovertheirthoughtstothosewho，beingusuallyintheircompany，haveleisuretolearntheirlanguage。
Andthisprovesnotonlythatthebruteshavelessreasonthanman，butthattheyhavenoneatall：forweseethatverylittleisrequiredtoenableapersontospeak；andsinceacertaininequalityofcapacityisobservableamonganimalsofthesamespecies，aswellasamongmen，andsincesomearemorecapableofbeinginstructedthanothers，itisincrediblethatthemostperfectapeorparrotofitsspecies，shouldnotinthisbeequaltothemoststupidinfantofitskindoratleasttoonethatwascrack—brained，unlessthesoulofbruteswereofanaturewhollydifferentfromours。Andweoughtnottoconfoundspeechwiththenaturalmovementswhichindicatethepassions，andcanbeimitatedbymachinesaswellasmanifestedbyanimals；normustitbethoughtwithcertainoftheancients，thatthebrutesspeak，althoughwedonotunderstandtheirlanguage。Forifsuchwerethecase，sincetheyareendowedwithmanyorgansanalogoustoours，theycouldaseasilycommunicatetheirthoughtstousastotheirfellows。Itisalsoveryworthyofremark，that，thoughtherearemanyanimalswhichmanifestmoreindustrythanweincertainoftheiractions，thesameanimalsareyetobservedtoshownoneatallinmanyothers：sothatthecircumstancethattheydobetterthanwedoesnotprovethattheyareendowedwithmind，foritwouldthencefollowthattheypossessedgreaterreasonthananyofus，andcouldsurpassusinallthings；onthecontrary，itratherprovesthattheyaredestituteofreason，andthatitisnaturewhichactsinthemaccordingtothedispositionoftheirorgans：thusitisseen，thataclockcomposedonlyofwheelsandweightscannumberthehoursandmeasuretimemoreexactlythanwewithallourskin。
Ihadafterthisdescribedthereasonablesoul，andshownthatitcouldbynomeansbeeducedfromthepowerofmatter，astheotherthingsofwhichIhadspoken，butthatitmustbeexpresslycreated；andthatitisnotsufficientthatitbelodgedinthehumanbodyexactlylikeapilotinaship，unlessperhapstomoveitsmembers，butthatitisnecessaryforittobejoinedandunitedmorecloselytothebody，inordertohavesensationsandappetitessimilartoours，andthusconstitutearealman。