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EVERYsystematicscience，thehumblestandthenoblestalike，seemstoadmitoftwodistinctkindsofproficiency；oneofwhichmaybeproperlycalledscientificknowledgeofthesubject，whiletheotherisakindofeducationalacquaintancewithit。Foraneducatedmanshouldbeabletoformafairoff-handjudgementastothegoodnessorbadnessofthemethodusedbyaprofessorinhisexposition。Tobeeducatedisinfacttobeabletodothis；andeventhemanofuniversaleducationwedeemtobesuchinvirtueofhishavingthisability。Itwill，however，ofcourse，beunderstoodthatweonlyascribeuniversaleducationtoonewhoinhisownindividualpersonisthuscriticalinallornearlyallbranchesofknowledge，andnottoonewhohasalikeabilitymerelyinsomespecialsubject。Foritispossibleforamantohavethiscompetenceinsomeonebranchofknowledgewithouthavingitinall。
Itisplainthenthat，asinothersciences，sointhatwhichinquiresintonature，theremustbecertaincanons，byreferencetowhichahearershallbeabletocriticizethemethodofaprofessedexposition，quiteindependentlyofthequestionwhetherthestatementsmadebetrueorfalse。Oughtwe，forinstance（togiveanillustrationofwhatImean），tobeginbydiscussingeachseparatespecies-man，lion，ox，andthelike-takingeachkindinhandinde。pendentlyoftherest，oroughtwerathertodealfirstwiththeattributeswhichtheyhaveincommoninvirtueofsomecommonelementoftheirnature，andproceedfromthisasabasisfortheconsiderationofthemseparately？Forgenerathatarequitedistinctyetoftentimespresentmanyidenticalphenomena，sleep，forinstance，respiration，growth，decay，death，andothersimilaraffectionsandconditions，whichmaybepassedoverforthepresent，aswearenotyetpreparedtotreatofthemwithclearnessandprecision。Nowitisplainthatifwedealwitheachspeciesindependentlyoftherest，weshallfrequentlybeobligedtorepeatthesamestatementsoverandoveragain；forhorseanddogandmanpresent，eachandall，everyoneofthephenomenajustenumerated。Adiscussionthereforeoftheattributesofeachsuchspeciesseparatelywouldnecessarilyinvolvefrequentrepetitionsastocharacters，themselvesidenticalbutrecurringinanimalsspecificallydistinct。（Verypossiblyalsotheremaybeothercharacterswhich，thoughtheypresentspecificdifferences，yetcomeunderoneandthesamecategory。Forinstance，flying，swimming，walking，creeping，areplainlyspecificallydistinct，butyetareallformsofanimalprogression。）Wemust，then，havesomeclearunderstandingastothemannerinwhichourinvestigationistobeconducted；whether，Imean，wearefirsttodealwiththecommonorgenericcharacters，andafterwardstotakeintoconsiderationspecialpeculiarities；orwhetherwearetostartstraightoffwiththeultimatespecies。Forasyetnodefiniterulehasbeenlaiddowninthismatter。Soalsothereisalikeuncertaintyastoanotherpointnowtobementioned。Oughtthewriterwhodealswiththeworksofnaturetofollowtheplanadoptedbythemathematiciansintheirastronomicaldemonstrations，andafterconsideringthephenomenapresentedbyanimals，andtheirseveralparts，proceedsubsequentlytotreatofthecausesandthereasonwhy；
oroughthetofollowsomeothermethod？Andwhenthesequestionsareanswered，thereyetremainsanother。Thecausesconcernedinthegenerationoftheworksofnatureare，aswesee，morethanone。Thereisthefinalcauseandthereisthemotorcause。Nowwemustdecidewhichofthesetwocausescomesfirst，whichsecond。Plainly，however，thatcauseisthefirstwhichwecallthefinalone。ForthisistheReason，andtheReasonformsthestarting-point，alikeintheworksofartandinworksofnature。Forconsiderhowthephysicianorhowthebuildersetsabouthiswork。Hestartsbyformingforhimselfadefinitepicture，intheonecaseperceptibletomind，intheothertosense，ofhisend-thephysicianofhealth，thebuilderofahouse-andthisheholdsforwardasthereasonandexplanationofeachsubsequentstepthathetakes，andofhisactinginthisorthatwayasthecasemaybe。Nowintheworksofnaturethegoodendandthefinalcauseisstillmoredominantthaninworksofartsuchasthese，norisnecessityafactorwiththesamesignificanceinthemall；thoughalmostallwriters，whiletheytrytorefertheirorigintothiscause，dosowithoutdistinguishingthevarioussensesinwhichthetermnecessityisused。Forthereisabsolutenecessity，manifestedineternalphenomena；andthereishypotheticalnecessity，manifestedineverythingthatisgeneratedbynatureasineverythingthatisproducedbyart，beitahouseorwhatitmay。
Forifahouseorothersuchfinalobjectistoberealized，itisnecessarythatsuchandsuchmaterialshallexist；anditisnecessarythatfirstthisthenthatshallbeproduced，andfirstthisandthenthatsetinmotion，andsoonincontinuoussuccession，untiltheendandfinalresultisreached，forthesakeofwhicheachpriorthingisproducedandexists。Aswiththeseproductionsofart，soalsoisitwiththeproductionsofnature。Themodeofnecessity，however，andthemodeofratiocinationaredifferentinnaturalsciencefromwhattheyareinthetheoreticalsciences；ofwhichwehavespokenelsewhere。Forinthelatterthestarting-pointisthatwhichis；intheformerthatwhichistobe。Foritisthatwhichisyettobe-health，letussay，oraman-that，owingtoitsbeingofsuchandsuchcharacters，necessitatesthepre-existenceorpreviousproductionofthisandthatantecedent；andnotthisorthatantecedentwhich，becauseitexistsorhasbeengenerated，makesitnecessarythathealthoramanisin，orshallcomeinto，existence。
Norisitpossibletotrackbacktheseriesofnecessaryantecedentstoastarting-point，ofwhichyoucansaythat，existingitselffrometernity，ithasdeterminedtheirexistenceasitsconsequent。Thesehoweveragain，aremattersthathavebeendealtwithinanothertreatise。Theretooitwasstatedinwhatcasesabsoluteandhypotheticalnecessityexist；inwhatcasesalsothepropositionexpressinghypotheticalnecessityissimplyconvertible，andwhatcauseitisthatdeterminesthisconvertibility。
Anothermatterwhichmustnotbepassedoverwithoutconsiderationis，whetherthepropersubjectofourexpositionisthatwithwhichtheancientwritersconcernedthemselves，namely，whatistheprocessofformationofeachanimal；orwhetheritisnotrather，whatarethecharactersofagivencreaturewhenformed。Forthereisnosmalldifferencebetweenthesetwoviews。Thebestcourseappearstobethatweshouldfollowthemethodalreadymentioned，andbeginwiththephenomenapresentedbyeachgroupofanimals，and，whenthisisdone，proceedafterwardstostatethecausesofthosephenomena，andtodealwiththeirevolution。Forelsewhere，asforinstanceinhousebuilding，thisisthetruesequence。Theplanofthehouse，orthehouse，hasthisandthatform；andbecauseithasthisandthatform，thereforeisitsconstructioncarriedoutinthisorthatmanner。FortheprocessofevolutionisforthesakeofthethingAnallyevolved，andnotthisforthesakeoftheprocess。
Empedocles，then，wasinerrorwhenhesaidthatmanyofthecharacterspresentedbyanimalsweremerelytheresultsofincidentaloccurrencesduringtheirdevelopment；forinstance，thatthebackbonewasdividedasitisintovertebrae，becauseithappenedtobebrokenowingtothecontortedpositionofthefoetusinthewomb。Insosayingheoverlookedthefactthatpropagationimpliesacreativeseedendowedwithcertainformativeproperties。Secondly，heneglectedanotherfact，namely，thattheparentanimalpre-exists，notonlyinidea，butactuallyintime。Formanisgeneratedfromman；andthusitisthepossessionofcertaincharactersbytheparentthatdeterminesthedevelopmentoflikecharactersinthechild。Thesamestatementholdsgoodalsofortheoperationsofart，andevenforthosewhichareapparentlyspontaneous。Forthesameresultasisproducedbyartmayoccurspontaneously。Spontaneity，forinstance，maybringabouttherestorationofhealth。Theproductsofart，however，requirethepre-existenceofanefficientcausehomogeneouswiththemselves，suchasthestatuary’sart，whichmustnecessarilyprecedethestatue；
forthiscannotpossiblybeproducedspontaneously。Artindeedconsistsintheconceptionoftheresulttobeproducedbeforeitsrealizationinthematerial。Aswithspontaneity，sowithchance；
forthisalsoproducesthesameresultasart，andbythesameprocess。
Thefittestmode，then，oftreatmentistosay，amanhassuchandsuchparts，becausetheconceptionofamanincludestheirpresence，andbecausetheyarenecessaryconditionsofhisexistence，or，ifwecannotquitesaythis，whichwouldbebestofall，thenthenextthingtoit，namely，thatitiseitherquiteimpossibleforhimtoexistwithoutthem，or，atanyrate，thatitisbetterforhimthattheyshouldbethere；andtheirexistenceinvolvestheexistenceofotherantecedents。Thusweshouldsay，becausemanisananimalwithsuchandsuchcharacters，thereforeistheprocessofhisdevelopmentnecessarilysuchasitis；andthereforeisitaccomplishedinsuchandsuchanorder，thispartbeingformedfirst，thatnext，andsooninsuccession；andafteralikefashionshouldweexplaintheevolutionofallotherworksofnature。
Nowthatwithwhichtheancientwriters，whofirstphilosophizedaboutNature，busiedthemselves，wasthematerialprincipleandthematerialcause。Theyinquiredwhatthisis，andwhatitscharacter；
howtheuniverseisgeneratedoutofit，andbywhatmotorinfluence，whether，forinstance，byantagonismorfriendship，whetherbyintelligenceorspontaneousaction，thesubstratumofmatterbeingassumedtohavecertaininseparableproperties；fire，forinstance，tohaveahotnature，earthacoldone；theformertobelight，thelatterheavy。Foreventhegenesisoftheuniverseisthusexplainedbythem。Afteralikefashiondotheydealalsowiththedevelopmentofplantsandofanimals。Theysay，forinstance，thatthewatercontainedinthebodycausesbyitscurrentstheformationofthestomachandtheotherreceptaclesoffoodorofexcretion；
andthatthebreathbyitspassagebreaksopentheoutletsofthenostrils；airandwaterbeingthematerialsofwhichbodiesaremade；forallrepresentnatureascomposedofsuchorsimilarsubstances。
Butifmenandanimalsandtheirseveralpartsarenaturalphenomena，thenthenaturalphilosophermusttakeintoconsiderationnotmerelytheultimatesubstancesofwhichtheyaremade，butalsoflesh，bone，blood，andallotherhomogeneousparts；notonlythese，butalsotheheterogeneousparts，suchasface，hand，foot；andmustexaminehoweachofthesecomestobewhatitis，andinvirtueofwhatforce。Fortosaywhataretheultimatesubstancesoutofwhichananimalisformed，tostate，forinstance，thatitismadeoffireorearth，isnomoresufficientthanwouldbeasimilaraccountinthecaseofacouchorthelike。Forweshouldnotbecontentwithsayingthatthecouchwasmadeofbronzeorwoodorwhateveritmightbe，butshouldtrytodescribeitsdesignormodeofcompositioninpreferencetothematerial；or，ifwediddealwiththematerial，itwouldatanyratebewiththeconcretionofmaterialandform。
Foracouchissuchandsuchaformembodiedinthisorthatmatter，orsuchandsuchamatterwiththisorthatform；sothatitsshapeandstructuremustbeincludedinourdescription。Fortheformalnatureisofgreaterimportancethanthematerialnature。
Does，then，configurationandcolourconstitutetheessenceofthevariousanimalsandoftheirseveralparts？Forifso，whatDemocritussayswillbestrictlycorrect。Forsuchappearstohavebeenhisnotion。Atanyratehesaysthatitisevidenttoeveryonewhatformitisthatmakestheman，seeingthatheisrecognizablebyhisshapeandcolour。Andyetadeadbodyhasexactlythesameconfigurationasalivingone；butforallthatisnotaman。Soalsonohandofbronzeorwoodorconstitutedinanybuttheappropriatewaycanpossiblybeahandinmorethanname。Forlikeaphysicianinapainting，orlikeafluteinasculpture，inspiteofitsnameitwillbeunabletodotheofficewhichthatnameimplies。
Preciselyinthesamewaynopartofadeadbody，suchImeanasitseyeoritshand，isreallyaneyeorahand。Tosay，then，thatshapeandcolourconstitutetheanimalisaninadequatestatement，andismuchthesameasifawoodcarverweretoinsistthatthehandhehadcutoutwasreallyahand。Yetthephysiologists，whentheygiveanaccountofthedevelopmentandcausesoftheanimalform，speakverymuchlikesuchacraftsman。What，however，Iwouldask，aretheforcesbywhichthehandorthebodywasfashionedintoitsshape？Thewoodcarverwillperhapssay，bytheaxeortheauger；thephysiologist，byairandbyearth。Ofthesetwoanswerstheartificer’sisthebetter，butitisneverthelessinsufficient。Foritisnotenoughforhimtosaythatbythestrokeofhistoolthispartwasformedintoaconcavity，thatintoaflatsurface；buthemuststatethereasonswhyhestruckhisblowinsuchawayastoeffectthis，andwhathisfinalobjectwas；namely，thatthepieceofwoodshoulddevelopeventuallyintothisorthatshape。Itisplain，then，thattheteachingoftheoldphysiologistsisinadequate，andthatthetruemethodistostatewhatthedefinitivecharactersarethatdistinguishtheanimalasawhole；toexplainwhatitisbothinsubstanceandinform，andtodealafterthesamefashionwithitsseveralorgans；infact，toproceedinexactlythesamewayasweshoulddo，werewegivingacompletedescriptionofacouch。
Ifnowthissomethingthatconstitutestheformofthelivingbeingbethesoul，orpartofthesoul，orsomethingthatwithoutthesoulcannotexist；aswouldseemtobethecase，seeingatanyratethatwhenthesouldeparts，whatisleftisnolongeralivinganimal，andthatnoneofthepartsremainwhattheywerebefore，exceptinginmereconfiguration，liketheanimalsthatinthefableareturnedintostone；if，Isay，thisbeso，thenitwillcomewithintheprovinceofthenaturalphilosophertoinformhimselfconcerningthesoul，andtotreatofit，eitherinitsentirety，or，atanyrate，ofthatpartofitwhichconstitutestheessentialcharacterofananimal；anditwillbehisdutytosaywhatthissoulorthispartofasoulis；andtodiscusstheattributesthatattachtothisessentialcharacter，especiallyasnatureisspokenofintwosenses，andthenatureofathingiseitheritsmatteroritsessence；
natureasessenceincludingboththemotorcauseandthefinalcause。Nowitisinthelatterofthesetwosensesthateitherthewholesoulorsomepartofitconstitutesthenatureofananimal；andinasmuchasitisthepresenceofthesoulthatenablesmattertoconstitutetheanimalnature，muchmorethanitisthepresenceofmatterwhichsoenablesthesoul，theinquirerintonatureisboundoneverygroundtotreatofthesoulratherthanofthematter。Forthoughthewoodofwhichtheyaremadeconstitutesthecouchandthetripod，itonlydoessobecauseitiscapableofreceivingsuchandsuchaform。
Whathasbeensaidsuggeststhequestion，whetheritisthewholesouloronlysomepartofit，theconsiderationofwhichcomeswithintheprovinceofnaturalscience。Nowifitbeofthewholesoulthatthisshouldtreat，thenthereisnoplaceforanyotherphilosophybesideit。Forasitbelongsinallcasestooneandthesamesciencetodealwithcorrelatedsubjects-oneandthesamescience，forinstance，dealswithsensationandwiththeobjectsofsense-andasthereforetheintelligentsoulandtheobjectsofintellect，beingcorrelated，mustbelongtooneandthesamescience，itfollowsthatnaturalsciencewillhavetoincludethewholeuniverseinitsprovince。Butperhapsitisnotthewholesoul，norallitspartscollectively，thatconstitutesthesourceofmotion；buttheremaybeonepart，identicalwiththatinplants，whichisthesourceofgrowth，another，namelythesensorypart，whichisthesourceofchangeofquality，whilestillanother，andthisnottheintellectualpart，isthesourceoflocomotion。Isaynottheintellectualpart；forotheranimalsthanmanhavethepoweroflocomotion，butinnonebuthimisthereintellect。Thusthenitisplainthatitisnotofthewholesoulthatwehavetotreat。Foritisnotthewholesoulthatconstitutestheanimalnature，butonlysomepartorpartsofit。Moreover，itisimpossiblethatanyabstractioncanformasubjectofnaturalscience，seeingthateverythingthatNaturemakesismeanstoanend。Forjustashumancreationsaretheproductsofart，solivingobjectsaremanifestintheproductsofananalogouscauseorprinciple，notexternalbutinternal，derivedlikethehotandthecoldfromtheenvironinguniverse。Andthattheheaven，ifithadanorigin，wasevolvedandismaintainedbysuchacause，thereisthereforeevenmorereasontobelieve，thanthatmortalanimalssooriginated。Fororderanddefinitenessaremuchmoreplainlymanifestinthecelestialbodiesthaninourownframe；whilechangeandchancearecharacteristicoftheperishablethingsofearth。Yettherearesomewho，whiletheyallowthateveryanimalexistsandwasgeneratedbynature，neverthelessholdthattheheavenwasconstructedtobewhatitisbychanceandspontaneity；theheaven，inwhichnotthefaintestsignofhaphazardorofdisorderisdiscernible！Again，wheneverthereisplainlysomefinalend，towhichamotiontendsshouldnothingstandintheway，wealwayssaythatsuchfinalendistheaimorpurposeofthemotion；andfromthisitisevidentthattheremustbeasomethingorotherreallyexisting，correspondingtowhatwecallbythenameofNature。Foragivengermdoesnotgiverisetoanychancelivingbeing，norspringfromanychanceone；buteachgermspringsfromadefiniteparentandgivesrisetoadefiniteprogeny。
Andthusitisthegermthatistherulinginfluenceandfabricatoroftheoffspring。Fortheseitisbynature，theoffspringbeingatanyratethatwhichinnaturewillspringfromit。Atthesametimetheoffspringisanteriortothegerm；forgermandperfectedprogenyarerelatedasthedevelopmentalprocessandtheresult。Anterior，however，tobothgermandproductistheorganismfromwhichthegermwasderived。Foreverygermimpliestwoorganisms，theparentandtheprogeny。Forgermorseedisboththeseedoftheorganismfromwhichitcame，ofthehorse，forinstance，fromwhichitwasderived，andtheseedoftheorganismthatwilleventuallyarisefromit，ofthemule，forexample，whichisdevelopedfromtheseedofthehorse。Thesameseedthenistheseedbothofthehorseandofthemule，thoughindifferentwaysasheresetforth。Moreover，theseedispotentiallythatwhichwillspringfromit，andtherelationofpotentialitytoactualityweknow。
Therearethentwocauses，namely，necessityandthefinalend。
Formanythingsareproduced，simplyastheresultsofnecessity。Itmay，however，beasked，ofwhatmodeofnecessityarewespeakingwhenwesaythis。Foritcanbeofneitherofthosetwomodeswhicharesetforthinthephilosophicaltreatises。Thereis，however，thethirdmode，insuchthingsatanyrateasaregenerated。Forinstance，wesaythatfoodisnecessary；becauseananimalcannotpossiblydowithoutit。Thisthirdmodeiswhatmaybecalledhypotheticalnecessity。Hereisanotherexampleofit。Ifapieceofwoodistobesplitwithanaxe，theaxemustofnecessitybehard；and，ifhard，mustofnecessitybemadeofbronzeoriron。Nowexactlyinthesamewaythebody，whichliketheaxeisaninstrument-forboththebodyasawholeanditsseveralpartsindividuallyhavedefiniteoperationsforwhichtheyaremade-justinthesameway，Isay，thebody，ifitistodoitswork，mustofnecessitybeofsuchandsuchacharacter，andmadeofsuchandsuchmaterials。
Itisplainthenthattherearetwomodesofcausation，andthatbothofthesemust，sofaraspossible，betakenintoaccountinexplainingtheworksofnature，orthatatanyrateanattemptmustbemadetoincludethemboth；andthatthosewhofailinthistellusinrealitynothingaboutnature。Forprimarycauseconstitutesthenatureofananimalmuchmorethandoesitsmatter。ThereareindeedpassagesinwhichevenEmpedocleshitsuponthis，andfollowingtheguidanceoffact，findshimselfconstrainedtospeakoftheratio（olugos）asconstitutingtheessenceandrealnatureofthings。
Such，forinstance，isthecasewhenheexplainswhatisabone。Forhedoesnotmerelydescribeitsmaterial，andsayitisthisoneelement，orthosetwoorthreeelements，oracompoundofalltheelements，butstatestheratio（olugos）oftheircombination。Aswithabone，somanifestlyisitwiththefleshandallothersimilarparts。
Thereasonwhyourpredecessorsfailedinhittinguponthismethodoftreatmentwas，thattheywerenotinpossessionofthenotionofessence，norofanydefinitionofsubstance。ThefirstwhocamenearitwasDemocritus，andhewasfarfromadoptingitasanecessarymethodinnaturalscience，butwasmerelybroughttoit，spiteofhimself，byconstraintoffacts。InthetimeofSocratesanearerapproachwasmadetothemethod。Butatthisperiodmengaveupinquiringintotheworksofnature，andphilosophersdivertedtheirattentiontopoliticalscienceandtothevirtueswhichbenefitmankind。
Ofthemethoditselfthefollowingisanexample。Indealingwithrespirationwemustshowthatittakesplaceforsuchorsuchafinalobject；andwemustalsoshowthatthisandthatpartoftheprocessisnecessitatedbythisandthatotherstageofit。Bynecessityweshallsometimesmeanhypotheticalnecessity，thenecessity，thatis，thattherequisiteantecedantsshallbethere，ifthefinalendistobereached；andsometimesabsolutenecessity，suchnecessityasthatwhichconnectssubstancesandtheirinherentpropertiesandcharacters。Forthealternatedischargeandre-entranceofheatandtheinflowofairarenecessaryifwearetolive。Herewehaveatonceanecessityintheformerofthetwosenses。Butthealternationofheatandrefrigerationproducesofnecessityanalternateadmissionanddischargeoftheouterair，andthisisanecessityofthesecondkind。
Intheforegoingwehaveanexampleofthemethodwhichwemustadopt，andalsoanexampleofthekindofphenomena，thecausesofwhichwehavetoinvestigate。
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Somewritersproposetoreachthedefinitionsoftheultimateformsofanimallifebybipartitedivision。Butthismethodisoftendifficult，andoftenimpracticable。
Sometimesthefinaldifferentiaofthesubdivisionissufficientbyitself，andtheantecedentdifferentiaearemeresurplusage。ThusintheseriesFooted，Two-footed，Cleft-footed，thelasttermisall-expressivebyitself，andtoappendthehighertermsisonlyanidleiteration。Againitisnotpermissibletobreakupanaturalgroup，Birdsforinstance，byputtingitsmembersunderdifferentbifurcations，asisdoneinthepublisheddichotomies，wheresomebirdsarerankedwithanimalsofthewater，andothersplacedinadifferentclass。ThegroupBirdsandthegroupFisheshappentobenamed，whileothernaturalgroupshavenopopularnames；forinstance，thegroupsthatwemaycallSanguineousandBloodlessarenotknownpopularlybyanydesignations。Ifsuchnaturalgroupsarenottobebrokenup，themethodofDichotomycannotbeemployed，foritnecessarilyinvolvessuchbreakingupanddislocation。ThegroupoftheMany-footed，forinstance，would，underthismethod，havetobedismembered，andsomeofitskindsdistributedamonglandanimals，othersamongwateranimals。
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Again，privativetermsinevitablyformonebranchofdichotomousdivision，asweseeintheproposeddichotomies。Butprivativetermsintheircharacterofprivativesadmitofnosubdivision。Fortherecanbenospecificformsofanegation，ofFeatherlessforinstanceorofFootless，asthereareofFeatheredandofFooted。Yetagenericdifferentiamustbesubdivisible；forotherwisewhatistherethatmakesitgenericratherthanspecific？Therearetobefoundgeneric，thatisspecificallysubdivisible，differentiae；FeatheredforinstanceandFooted。ForfeathersaredivisibleintoBarbedandUnbarbed，andfeetintoManycleft，andTwocleft，likethoseofanimalswithbifidhoofs，andUncleftorUndivided，likethoseofanimalswithsolidhoofs。Nowevenwithdifferentiaecapableofthisspecificsubdivisionitisdifficultenoughsotomaketheclassification，asthateachanimalshallbecomprehendedinsomeonesubdivisionandinnotmorethanone；butfarmoredifficult，nayimpossible，isittodothis，ifwestartwithadichotomyintotwocontradictories。
（Supposeforinstancewestartwiththetwocontradictories，FeatheredandUnfeathered；weshallfindthattheant，theglow-worm，andsomeotheranimalsfallunderbothdivisions。）Foreachdifferentiamustbepresentedbysomespecies。Theremustbesomespecies，therefore，undertheprivativeheading。Nowspecificallydistinctanimalscannotpresentintheiressenceacommonundifferentiatedelement，butanyapparentlycommonelementmustreallybedifferentiated。（BirdandManforinstancearebothTwo-footed，buttheirtwo-footednessisdiverseanddifferentiated。Soanytwosanguineousgroupsmusthavesomedifferenceintheirblood，iftheirbloodispartoftheiressence。）Fromthisitfollowsthataprivativeterm，beinginsusceptibleofdifferentiation，cannotbeagenericdifferentia；
for，ifitwere，therewouldbeacommonundifferentiatedelementintwodifferentgroups。
Again，ifthespeciesareultimateindivisiblegroups，thatis，aregroupswithindivisibledifferentiae，andifnodifferentiabecommontoseveralgroups，thenumberofdifferentiaemustbeequaltothenumberofspecies。Ifadifferentiathoughnotdivisiblecouldyetbecommontoseveralgroups，thenitisplainthatinvirtueofthatcommondifferentiaspecificallydistinctanimalswouldfallintothesamedivision。Itisnecessarythen，ifthedifferentiae，underwhicharerangedalltheultimateandindivisiblegroups，arespecificcharacters，thatnoneofthemshallbecommon；forotherwise，asalreadysaid，specificallydistinctanimalswillcomeintooneandthesamedivision。Butthiswouldviolateoneoftherequisiteconditions，whichareasfollows。Noultimategroupmustbeincludedinmorethanasingledivision；differentgroupsmustnotbeincludedinthesamedivision；andeverygroupmustbefoundinsomedivision。Itisplainthenthatwecannotgetattheultimatespecificformsoftheanimal，oranyother，kingdombybifurcatedivision。Ifwecould，thenumberofultimatedifferentiaewouldequalthenumberofultimateanimalforms。ForassumeanorderofbeingswhoseprimedifferentiaeareWhiteandBlack。Eachofthesebrancheswillbifurcate，andtheirbranchesagain，andsoontillwereachtheultimatedifferentiae，whosenumberwillbefourorsomeotherpoweroftwo，andwillalsobethenumberoftheultimatespeciescomprehendedintheorder。
（Aspeciesisconstitutedbythecombinationdifferentiaandmatter。
Fornopartofananimalispurelymaterialorpurelyimmaterial；
norcanabody，independentlyofitscondition，constituteananimaloranyofitsparts，ashasrepeatedlybeenobserved。）
Further，thedifferentiaemustbeelementsoftheessence，andnotmerelyessentialattributes。ThusifFigureisthetermtobedivided，itmustnotbedividedintofigureswhoseanglesareequaltotworightangles，andfigureswhoseanglesaretogethergreaterthantworightangles。Foritisonlyanattributeofatriangleandnotpartofitsessencethatitsanglesareequaltotworightangles。
Again，thebifurcationsmustbeopposites，likeWhiteandBlack，StraightandBent；andifwecharacterizeonebranchbyeitherterm，wemustcharacterizetheotherbyitsopposite，andnot，forexample，characterizeonebranchbyacolour，theotherbyamodeofprogression，swimmingforinstance。
Furthermore，livingbeingscannotbedividedbythefunctionscommontobodyandsoul，byFlying，forinstance，andWalking，asweseethemdividedinthedichotomiesalreadyreferredto。Forsomegroups，Antsforinstance，fallunderbothdivisions，someantsflyingwhileothersdonot。SimilarlyasregardsthedivisionintoWildandTame；
foritalsowouldinvolvethedisruptionofaspeciesintodifferentgroups。Forinalmostallspeciesinwhichsomemembersaretame，thereareothermembersthatarewild。Such，forexample，isthecasewithMen，Horses，Oxen，DogsinIndia，Pigs，Goats，Sheep；groupswhich，ifdouble，oughttohavewhattheyhavenot，namely，differentappellations；andwhich，ifsingle，provethatWildnessandTamenessdonotamounttospecificdifferences。Andwhateversingleelementwetakeasabasisofdivisionthesamedifficultywilloccur。
Themethodthenthatwemustadoptistoattempttorecognizethenaturalgroups，followingtheindicationsaffordedbytheinstinctsofmankind，whichledthemforinstancetoformtheclassofBirdsandtheclassofFishes，eachofwhichgroupscombinesamultitudeofdifferentiae，andisnotdefinedbyasingleoneasindichotomy。
Themethodofdichotomyiseitherimpossible（foritwouldputasinglegroupunderdifferentdivisionsorcontrarygroupsunderthesamedivision），oritonlyfurnishesasingleultimatedifferentiaforeachspecies，whicheitheraloneorwithitsseriesofantecedentshastoconstitutetheultimatespecies。
If，again，anewdifferentialcharacterbeintroducedatanystageintothedivision，thenecessaryresultisthatthecontinuityofthedivisionbecomesmerelyaunityandcontinuityofagglomeration，liketheunityandcontinuityofaseriesofsentencescoupledtogetherbyconjunctiveparticles。Forinstance，supposewehavethebifurcationFeatheredandFeatherless，andthendivideFeatheredintoWildandTame，orintoWhiteandBlack。TameandWhitearenotadifferentiationofFeathered，butarethecommencementofanindependentbifurcation，andareforeigntotheseriesattheendofwhichtheyareintroduced。
Aswesaidthen，wemustdefineattheoutsetbymultiplicityofdifferentiae。Ifwedoso，privativetermswillbeavailable，whichareunavailabletothedichotomist。
Theimpossibilityofreachingthedefinitionofanyoftheultimateformsbydichotomyofthelargergroup，assomepropose，ismanifestalsofromthefollowingconsiderations。Itisimpossiblethatasingledifferentia，eitherbyitselforwithitsantecedents，shallexpressthewholeessenceofaspecies。（InsayingasingledifferentiabyitselfImeansuchanisolateddifferentiaasCleft-footed；insayingasingledifferentiawithantecedentImean，togiveaninstance，Manycleft-footedprecededbyCleft-footed。Theverycontinuityofaseriesofsuccessivedifferentiaeinadivisionisintendedtoshowthatitistheircombinationthatexpressesthecharacteroftheresultingunit，orultimategroup。Butoneismisledbytheusagesoflanguageintoimaginingthatitismerelythefinaltermoftheseries，Manycleft-footedforinstance，thatconstitutesthewholedifferentia，andthattheantecedentterms，Footed，Cleft-footed，aresuperfluous。Nowitisevidentthatsuchaseriescannotconsistofmanyterms。Forifonedividesandsubdivides，onesoonreachesthefinaldifferentialterm，butforallthatwillnothavegottotheultimatedivision，thatis，tothespecies。）Nosingledifferentia，Irepeat，eitherbyitselforwithitsantecedents，canpossiblyexpresstheessenceofaspecies。
Suppose，forexample，Mantobetheanimaltobedefined；thesingledifferentiawillbeCleft-footed，eitherbyitselforwithitsantecedents，FootedandTwo-footed。NowifmanwasnothingmorethanaCleft-footedanimal，thissingledifferentiawoulddulyrepresenthisessence。Butseeingthatthisisnotthecase，moredifferentiaethanthisonewillnecessarilyberequiredtodefinehim；andthesecannotcomeunderonedivision；foreachsinglebranchofadichotomyendsinasingledifferentia，andcannotpossiblyincludeseveraldifferentiaebelongingtooneandthesameanimal。
Itisimpossiblethentoreachanyoftheultimateanimalformsbydichotomousdivision。
4
Itdeservesinquirywhyasinglenamedenotingahighergroupwasnotinventedbymankind，asanappellationtocomprehendthetwogroupsofWateranimalsandWingedanimals。Foreventhesehavecertainattributesincommon。However，thepresentnomenclatureisjust。Groupsthatonlydifferindegree，andinthemoreorlessofanidenticalelementthattheypossess，areaggregatedunderasingleclass；groupswhoseattributesarenotidenticalbutanalogousareseparated。Forinstance，birddiffersfrombirdbygradation，orbyexcessanddefect；somebirdshavelongfeathers，othersshortones，butallarefeathered。BirdandFisharemoreremoteandonlyagreeinhavinganalogousorgans；forwhatinthebirdisfeather，inthefishisscale。Suchanalogiescanscarcely，however，serveuniversallyasindicationsfortheformationofgroups，foralmostallanimalspresentanalogiesintheircorrespondingparts。
Theindividualscomprisedwithinaspecies，suchasSocratesandCoriscus，aretherealexistences；butinasmuchastheseindividualspossessonecommonspecificform，itwillsufficetostatetheuniversalattributesofthespecies，thatis，theattributescommontoallitsindividuals，onceforall，asotherwisetherewillbeendlessreiteration，ashasalreadybeenpointedout。
Butasregardsthelargergroups-suchasBirds-whichcomprehendmanyspecies，theremaybeaquestion。Forontheonehanditmaybeurgedthatastheultimatespeciesrepresenttherealexistences，itwillbewell，ifpracticable，toexaminetheseultimatespeciesseparately，justasweexaminethespeciesManseparately；toexamine，thatis，notthewholeclassBirdscollectively，buttheOstrich，theCrane，andtheotherindivisiblegroupsorspeciesbelongingtotheclass。
Ontheotherhand，however，thiscoursewouldinvolverepeatedmentionofthesameattribute，asthesameattributeiscommontomanyspecies，andsofarwouldbesomewhatirrationalandtedious。Perhaps，then，itwillbebesttotreatgenericallytheuniversalattributesofthegroupsthathaveacommonnatureandcontaincloselyalliedsubordinateforms，whethertheyaregroupsrecognizedbyatrueinstinctofmankind，suchasBirdsandFishes，orgroupsnotpopularlyknownbyacommonappellation，butwithalcomposedofcloselyalliedsubordinategroups；andonlytodealindividuallywiththeattributesofasinglespecies，whensuchspecies，man，forinstance，andanyothersuch，ifsuchtherebe-standsapartfromothers，anddoesnotconstitutewiththemalargernaturalgroup。
Itisgenerallysimilarityintheshapeofparticularorgans，orofthewholebody，thathasdeterminedtheformationofthelargergroups。ItisinvirtueofsuchasimilaritythatBirds，Fishes，Cephalopoda，andTestaceahavebeenmadetoformeachaseparateclass。Forwithinthelimitsofeachsuchclass，thepartsdonotdifferinthattheyhavenonearerresemblancethanthatofanalogy-suchasexistsbetweentheboneofmanandthespineoffish-butdiffermerelyinrespectofsuchcorporealconditionsaslargenesssmallness，softnesshardness，smoothnessroughness，andothersimilaroppositions，or，inoneword，inrespectofdegree。
Wehavenowtoucheduponthecanonsforcriticizingthemethodofnaturalscience，andhaveconsideredwhatisthemostsystematicandeasycourseofinvestigation；wehavealsodealtwithdivision，andthemodeofconductingitsoasbesttoattaintheendsofscience，andhaveshownwhydichotomyiseitherimpracticableorinefficaciousforitsprofessedpurposes。
Havinglaidthisfoundation，letuspassontoournexttopic。
5
Ofthingsconstitutedbynaturesomeareungenerated，imperishable，andeternal，whileothersaresubjecttogenerationanddecay。Theformerareexcellentbeyondcompareanddivine，butlessaccessibletoknowledge。Theevidencethatmightthrowlightonthem，andontheproblemswhichwelongtosolverespectingthem，isfurnishedbutscantilybysensation；whereasrespectingperishableplantsandanimalswehaveabundantinformation，livingaswedointheirmidst，andampledatamaybecollectedconcerningalltheirvariouskinds，ifonlywearewillingtotakesufficientpains。Bothdepartments，however，havetheirspecialcharm。Thescantyconceptionstowhichwecanattainofcelestialthingsgiveus，fromtheirexcellence，morepleasurethanallourknowledgeoftheworldinwhichwelive；justasahalfglimpseofpersonsthatweloveismoredelightfulthanaleisurelyviewofotherthings，whatevertheirnumberanddimensions。Ontheotherhand，incertitudeandincompletenessourknowledgeofterrestrialthingshastheadvantage。
Moreover，theirgreaternearnessandaffinitytousbalancessomewhattheloftierinterestoftheheavenlythingsthataretheobjectsofthehigherphilosophy。Havingalreadytreatedofthecelestialworld，asfarasourconjecturescouldreach，weproceedtotreatofanimals，withoutomitting，tothebestofourability，anymemberofthekingdom，howeverignoble。Forifsomehavenogracestocharmthesense，yeteventhese，bydisclosingtointellectualperceptiontheartisticspiritthatdesignedthem，giveimmensepleasuretoallwhocantracelinksofcausation，andareinclinedtophilosophy。Indeed，itwouldbestrangeifmimicrepresentationsofthemwereattractive，becausetheydisclosethemimeticskillofthepainterorsculptor，andtheoriginalrealitiesthemselveswerenotmoreinteresting，toallatanyratewhohaveeyestodiscernthereasonsthatdeterminedtheirformation。Wethereforemustnotrecoilwithchildishaversionfromtheexaminationofthehumbleranimals。Everyrealmofnatureismarvellous：andasHeraclitus，whenthestrangerswhocametovisithimfoundhimwarminghimselfatthefurnaceinthekitchenandhesitatedtogoin，reportedtohavebiddenthemnottobeafraidtoenter，aseveninthatkitchendivinitieswerepresent，soweshouldventureonthestudyofeverykindofanimalwithoutdistaste；foreachandallwillrevealtoussomethingnaturalandsomethingbeautiful。AbsenceofhaphazardandconducivenessofeverythingtoanendaretobefoundinNature’sworksinthehighestdegree，andtheresultantendofhergenerationsandcombinationsisaformofthebeautiful。
Ifanypersonthinkstheexaminationoftherestoftheanimalkingdomanunworthytask，hemustholdinlikedisesteemthestudyofman。Fornoonecanlookattheprimordiaofthehumanframe-blood，flesh，bones，vessels，andthelike-withoutmuchrepugnance。Moreover，whenanyoneofthepartsorstructures，beitwhichitmay，isunderdiscussion，itmustnotbesupposedthatitisitsmaterialcompositiontowhichattentionisbeingdirectedorwhichistheobjectofthediscussion，buttherelationofsuchparttothetotalform。Similarly，thetrueobjectofarchitectureisnotbricks，mortar，ortimber，butthehouse；andsotheprincipalobjectofnaturalphilosophyisnotthematerialelements，buttheircomposition，andthetotalityoftheform，independentlyofwhichtheyhavenoexistence。
Thecourseofexpositionmustbefirsttostatetheattributescommontowholegroupsofanimals，andthentoattempttogivetheirexplanation。Manygroups，asalreadynoticed，presentcommonattributes，thatistosay，insomecasesabsolutelyidenticalaffections，andabsolutelyidenticalorgans，-feet，feathers，scales，andthelike-whileinothergroupstheaffectionsandorgansareonlysofaridenticalasthattheyareanalogous。Forinstance，somegroupshavelungs，othershavenolung，butanorgananalogoustoalunginitsplace；somehaveblood，othershavenoblood，butafluidanalogoustoblood，andwiththesameoffice。Totreatofthecommonattributesinconnexionwitheachindividualgroupwouldinvolve，asalreadysuggested，uselessiteration。Formanygroupshavecommonattributes。Somuchforthistopic。
Aseveryinstrumentandeverybodilymembersubservessomepartialend，thatistosay，somespecialaction，sothewholebodymustbedestinedtoministertosomePlenarysphereofaction。Thusthesawismadeforsawing，forsawingisafunction，andnotsawingforthesaw。
Similarly，thebodytoomustsomehoworotherbemadeforthesoul，andeachpartofitforsomesubordinatefunction，towhichitisadapted。
Wehave，then，firsttodescribethecommonfunctions，common，thatis，tothewholeanimalkingdom，ortocertainlargegroups，ortothemembersofaspecies。Inotherwords，wehavetodescribetheattributescommontoallanimals，ortoassemblages，liketheclassofBirds，ofcloselyalliedgroupsdifferentiatedbygradation，ortogroupslikeMannotdifferentiatedintosubordinategroups。Inthefirstcasethecommonattributesmaybecalledanalogous，inthesecondgeneric，inthethirdspecific。
Whenafunctionisancillarytoanother，alikerelationmanifestlyobtainsbetweentheorganswhichdischargethesefunctions；
andsimilarly，ifonefunctionispriortoandtheendofanother，theirrespectiveorganswillstandtoeachotherinthesamerelation。
Thirdly，theexistenceofthesepartsinvolvesthatofotherthingsastheirnecessaryconsequents。
InstancesofwhatImeanbyfunctionsandaffectionsareReproduction，Growth，Copulation，Waking，Sleep，Locomotion，andothersimilarvitalactions。InstancesofwhatImeanbypartsareNose，Eye，Face，andotherso-calledmembersorlimbs，andalsothemoreelementarypartsofwhichthesearemade。Somuchforthemethodtobepursued。Letusnowtrytosetforththecausesofallvitalphenomena，whetheruniversalorparticular，andinsodoingletusfollowthatorderofexpositionwhichconforms，aswehaveindicated，totheorderofnature。
BookII
1
THEnatureandthenumberofthepartsofwhichanimalsareseverallycomposedarematterswhichhavealreadybeensetforthindetailinthebookofResearchesaboutAnimals。Wehavenowtoinquirewhatarethecausesthatineachcasehavedeterminedthiscomposition，asubjectquitedistinctfromthatdealtwithintheResearches。
Nowtherearethreedegreesofcomposition；andofthesethefirstinorder，asallwillallow，iscompositionoutofwhatsomecalltheelements，suchasearth，air，water，fire。Perhaps，however，itwouldbemoreaccuratetosaycompositionoutoftheelementaryforces；norindeedoutofallofthese，butoutofalimitednumberofthem，asdefinedinprevioustreatises。Forfluidandsolid，hotandcold，formthematerialofallcompositebodies；andallotherdifferencesaresecondarytothese，suchdifferences，thatis，asheavinessorlightness，densityorrarity，roughnessorsmoothness，andanyothersuchpropertiesofmatterastheremaybe。seconddegreeofcompositionisthatbywhichthehomogeneouspartsofanimals，suchasbone，flesh，andthelike，areconstitutedoutoftheprimarysubstances。Thethirdandlaststageisthecompositionwhichformstheheterogeneousparts，suchasface，hand，andtherest。
Nowtheorderofactualdevelopmentandtheorderoflogicalexistencearealwaystheinverseofeachother。Forthatwhichisposteriorintheorderofdevelopmentisantecedentintheorderofnature，andthatisgeneticallylastwhichinnatureisfirst。
（Thatthisissoismanifestbyinduction；forahousedoesnotexistforthesakeofbricksandstones，butthesematerialsforthesakeofthehouse；andthesameisthecasewiththematerialsofotherbodies。Norisinductionrequiredtoshowthis。itisincludedinourconceptionofgeneration。Forgenerationisaprocessfromasomethingtoasomething；thatwhichisgeneratedhavingacauseinwhichitoriginatesandacauseinwhichitends。Theoriginatingcauseistheprimaryefficientcause，whichissomethingalreadyendowedwithtangibleexistence，whilethefinalcauseissomedefiniteformorsimilarend；formangeneratesman，andplantgeneratesplant，ineachcaseoutoftheunderlyingmaterial。）
Inorderoftime，then，thematerialandthegenerativeprocessmustnecessarilybeanteriortothebeingthatisgenerated；butinlogicalorderthedefinitivecharacterandformofeachbeingprecedesthematerial。Thisisevidentifoneonlytriestodefinetheprocessofformation。Forthedefinitionofhouse-buildingincludesandpresupposesthatofthehouse；butthedefinitionofthehousedoesnotincludenorpresupposethatofhouse-building；andthesameistrueofallotherproductions。Sothatitmustnecessarilybethattheelementarymaterialexistsforthesakeofthehomogeneousparts，seeingthatthesearegeneticallyposteriortoit，justastheheterogeneouspartsareposteriorgeneticallytothem。Fortheseheterogeneouspartshavereachedtheendandgoal，havingthethirddegreeofcomposition，inwhichdegreegenerationordevelopmentoftenattainsitsfinalterm。
Animals，then，arecomposedofhomogeneousparts，andarealsocomposedofheterogeneousparts。Theformer，however，existforthesakeofthelatter。Fortheactivefunctionsandoperationsofthebodyarecarriedonbythese；thatis，bytheheterogeneousparts，suchastheeye，thenostril，thewholeface，thefingers，thehand，andthewholearm。Butinasmuchasthereisagreatvarietyinthefunctionsandmotionsnotonlyofaggregateanimalsbutalsooftheindividualorgans，itisnecessarythatthesubstancesoutofwhichthesearecomposedshallpresentadiversityofproperties。Forsomepurposessoftnessisadvantageous，forothershardness；somepartsmustbecapableofextension，othersofflexion。Suchproperties，then，aredistributedseparatelytothedifferenthomogeneousparts，onebeingsoftanotherhard，onefluidanothersolid，oneviscousanotherbrittle；whereaseachoftheheterogeneouspartspresentsacombinationofmultifariousproperties。Forthehand，totakeanexample，requiresonepropertytoenableittoeffectpressure，andanotheranddifferentpropertyforsimpleprehension。Forthisreasontheactiveorexecutivepartsofthebodyarecompoundedoutofbones，sinews，flesh，andthelike，butnottheselatteroutoftheformer。