posterior analytics-第18节

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against　a　doubt　as　to　its　having　been　a　syllogistic　inference　at　all



that　we　have　to　defend　our　argument　as　conforming　to　the　definition　of



syllogism。　It　is　only　when　some　one　doubts　whether　the　conclusion



proved　is　the　definable　form　that　we　have　to　defend　it　as　conforming



to　the　definition　of　definable　form　which　we　assumed。　Hence



syllogistic　inference　must　be　possible　even　without　the　express



statement　of　what　syllogism　is　or　what　definable　form　is。



　　The　following　type　of　hypothetical　proof　also　begs　the　question。



If　evil　is　definable　as　the　divisible；　and　the　definition　of　a　thing's



contrary…if　it　has　one　the　contrary　of　the　thing's　definition；　then；



if　good　is　the　contrary　of　evil　and　the　indivisible　of　the



divisible；　we　conclude　that　to　be　good　is　essentially　to　be



indivisible。　The　question　is　begged　because　definable　form　is



assumed　as　a　premiss；　and　as　a　premiss　which　is　to　prove　definable



form。　'But　not　the　same　definable　form'；　you　may　object。　That　I　admit；



for　in　demonstrations　also　we　premise　that　'this'　is　predicable　of



'that'；　but　in　this　premiss　the　term　we　assert　of　the　minor　is　neither



the　major　itself　nor　a　term　identical　in　definition；　or　convertible；



with　the　major。



　　Again；　both　proof　by　division　and　the　syllogism　just　described　are



open　to　the　question　why　man　should　be　animal…biped…terrestrial　and



not　merely　animal　and　terrestrial；　since　what　they　premise　does　not



ensure　that　the　predicates　shall　constitute　a　genuine　unity　and　not



merely　belong　to　a　single　subject　as　do　musical　and　grammatical　when



predicated　of　the　same　man。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　7







　　How　then　by　definition　shall　we　prove　substance　or　essential　nature？



We　cannot　show　it　as　a　fresh　fact　necessarily　following　from　the



assumption　of　premisses　admitted　to　be　facts…the　method　of



demonstration：　we　may　not　proceed　as　by　induction　to　establish　a



universal　on　the　evidence　of　groups　of　particulars　which　offer　no



exception；　because　induction　proves　not　what　the　essential　nature　of　a



thing　is　but　that　it　has　or　has　not　some　attribute。　Therefore；　since



presumably　one　cannot　prove　essential　nature　by　an　appeal　to　sense



perception　or　by　pointing　with　the　finger；　what　other　method　remains？



　　To　put　it　another　way：　how　shall　we　by　definition　prove　essential



nature？　He　who　knows　what　human…or　any　other…nature　is；　must　know　also



that　man　exists；　for　no　one　knows　the　nature　of　what　does　not



exist…one　can　know　the　meaning　of　the　phrase　or　name　'goat…stag'　but



not　what　the　essential　nature　of　a　goat…stag　is。　But　further；　if



definition　can　prove　what　is　the　essential　nature　of　a　thing；　can　it



also　prove　that　it　exists？　And　how　will　it　prove　them　both　by　the　same



process；　since　definition　exhibits　one　single　thing　and



demonstration　another　single　thing；　and　what　human　nature　is　and　the



fact　that　man　exists　are　not　the　same　thing？　Then　too　we　hold　that



it　is　by　demonstration　that　the　being　of　everything　must　be



proved…unless　indeed　to　be　were　its　essence；　and；　since　being　is　not　a



genus；　it　is　not　the　essence　of　anything。　Hence　the　being　of



anything　as　fact　is　matter　for　demonstration；　and　this　is　the　actual



procedure　of　the　sciences；　for　the　geometer　assumes　the　meaning　of　the



word　triangle；　but　that　it　is　possessed　of　some　attribute　he　proves。



What　is　it；　then；　that　we　shall　prove　in　defining　essential　nature？



Triangle？　In　that　case　a　man　will　know　by　definition　what　a　thing's



nature　is　without　knowing　whether　it　exists。　But　that　is　impossible。



　　Moreover　it　is　clear；　if　we　consider　the　methods　of　defining



actually　in　use；　that　definition　does　not　prove　that　the　thing　defined



exists：　since　even　if　there　does　actually　exist　something　which　is



equidistant　from　a　centre；　yet　why　should　the　thing　named　in　the



definition　exist？　Why；　in　other　words；　should　this　be　the　formula



defining　circle？　One　might　equally　well　call　it　the　definition　of



mountain　copper。　For　definitions　do　not　carry　a　further　guarantee　that



the　thing　defined　can　exist　or　that　it　is　what　they　claim　to　define：



one　can　always　ask　why。



　　Since；　therefore；　to　define　is　to　prove　either　a　thing's　essential



nature　or　the　meaning　of　its　name；　we　may　conclude　that　definition；　if



it　in　no　sense　proves　essential　nature；　is　a　set　of　words　signifying



precisely　what　a　name　signifies。　But　that　were　a　strange



consequence；　for　（1）　both　what　is　not　substance　and　what　does　not



exist　at　all　would　be　definable；　since　even　non…existents　can　be



signified　by　a　name：　（2）　all　sets　of　words　or　sentences　would　be



definitions；　since　any　kind　of　sentence　could　be　given　a　name；　so　that



we　should　all　be　talking　in　definitions；　and　even　the　Iliad　would　be　a



definition：　（3）　no　demonstration　can　prove　that　any　particular　name



means　any　particular　thing：　neither；　therefore；　do　definitions；　in



addition　to　revealing　the　meaning　of　a　name；　also　reveal　that　the　name



has　this　meaning。　It　appears　then　from　these　considerations　that



neither　definition　and　syllogism　nor　their　objects　are　identical；



and　further　that　definition　neither　demonstrates　nor　proves



anything；　and　that　knowledge　of　essential　nature　is　not　to　be　obtained



either　by　definition　or　by　demonstration。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　8







　　We　must　now　start　afresh　and　consider　which　of　these　conclusions　are



sound　and　which　are　not；　and　what　is　the　nature　of　definition；　and



whether　essential　nature　is　in　any　sense　demonstrable　and　definable　or



in　none。



　　Now　to　know　its　essential　nature　is；　as　we　said；　the　same　as　to　know



the　cause　of　a　thing's　existence；　and　the　proof　of　this　depends　on　the



fact　that　a　thing　must　have　a　cause。　Moreover；　this　cause　is　either



identical　with　the　essential　nature　of　the　thing　or　distinct　from



it；　and　if　its　cause　is　distinct　from　it；　the　essential　nature　of



the　thing　is　either　demonstrable　or　indemonstrable。　Consequently；　if



the　cause　is　distinct　from　the　thing's　essential　nature　and



demonstration　is　possible；　the　cause　must　be　the　middle　term；　and；　the



conclusion　proved　being　universal　and　affirmative；　the　proof　is　in　the



first　figure。　So　the　method　just　examined　of　proving　it　through



another　essential　nature　would　be　one　way　of　proving　essential　nature；



because　a　conclusion　containing　essential　nature　must　be　inferred



through　a　middle　which　is　an　essential　nature　just　as　a　'peculiar'



property　must　be　inferred　through　a　middle　which　is　a　'peculiar'



property；　so　that　of　the　two　definable　natures　of　a　single　thing



this　method　will　prove　one　and　not　the　other。



　　Now　it　was　said　before　that　this　method　could　not　amount　to



demonstration　of　essential　nature…it　is　actually　a　dialectical　proof



of　it…so　let　us　begin　again　and　explain　by　what　method　it　can　be



demonstrated。　When　we　are　aware　of　a　fact　we　seek　its　reason；　and



though　sometimes　the　fact　and　the　reason　dawn　on　us　simultaneously；



yet　we　cannot　apprehend　the　reason　a　moment　sooner　than　the　fact；



and　clearly　in　just　the　same　way　we　cannot　apprehend　a　thing's



definable　form　without　apprehending　that　it　exists；　since　while　we　are



ignorant　whether　it　exists　we　cannot　know　its　essential　nature。



Moreover　we　are　aware　whether　a　thing　exists　or　not　sometimes



through　apprehending　an　element　in　its　character；　and　sometimes



accidentally；　as；　for　example；　when　we　are　aware　of　thunder　as　a　noise



in　the　clouds；　of　eclipse　as　a　privation　of　light；　or　of　man　as　some



species　of　animal；　or　of　the　soul　as　a　self…moving　thing。　As　often



as　we　have　accidental　knowledge　that　the　thing　exists；　we　must　be　in　a



wholly　negative　state　as　regards　awareness　of　its　essential　nature；



for　we　have　not　got　genuine　knowledge　even　of　its　existence；　and　to



search　for　a　thing's　essential　nature　when　we　are　unaware　that　it



exists　is　to　search　for　nothing。　On　the　other　hand；　whenever　we



apprehend　an　element　in　the　thing's　character　there　is　less



difficulty。　Thus　it　follows　that　the　degree　of　our　knowledge　of　a



thing's　essential　nature　is　determined　by　the　sense　in　which　we　are



aware　that　it　exists。　Let　us　then　take　the　following　as　our　first



instance　of　being　aware　of　an　element　in　the　essential　nature。　Let　A



be　eclipse；　C　the　moon；　B　the　earth's　acting　as　a　screen。　Now　to　ask



whether　the　moon　is　eclipsed　or　not　is　to　ask　whether　or　not　B　has



occurred。　But　that　is　precisely　the　same　as　asking　whether　A　has　a



defining　condition；　and　if　this　condition　actually　exists；　we　assert



that　A　also　actually　exists。　Or　again