posterior analytics- 第17部分

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　we　may　ask　which　side　of　a



contradiction　the　defining　condition　necessitates：　does　it　make　the



angles　of　a　triangle　equal　or　not　equal　to　two　right　angles？　When　we



have　found　the　answer；　if　the　premisses　are　immediate；　we　know　fact



and　reason　together；　if　they　are　not　immediate；　we　know　the　fact



without　the　reason；　as　in　the　following　example：　let　C　be　the　moon；



A　eclipse；　B　the　fact　that　the　moon　fails　to　produce　shadows　though



she　is　full　and　though　no　visible　body　intervenes　between　us　and



her。　Then　if　B；　failure　to　produce　shadows　in　spite　of　the　absence



of　an　intervening　body；　is　attributable　A　to　C；　and　eclipse；　is



attributable　to　B；　it　is　clear　that　the　moon　is　eclipsed；　but　the



reason　why　is　not　yet　clear；　and　we　know　that　eclipse　exists；　but　we



do　not　know　what　its　essential　nature　is。　But　when　it　is　clear　that



A　is　attributable　to　C　and　we　proceed　to　ask　the　reason　of　this



fact；　we　are　inquiring　what　is　the　nature　of　B：　is　it　the　earth's



acting　as　a　screen；　or　the　moon's　rotation　or　her　extinction？　But　B　is



the　definition　of　the　other　term；　viz。　in　these　examples；　of　the　major



term　A；　for　eclipse　is　constituted　by　the　earth　acting　as　a　screen。



Thus；　（1）　'What　is　thunder？'　'The　quenching　of　fire　in　cloud'；　and　（2）



'Why　does　it　thunder？'　'Because　fire　is　quenched　in　the　cloud'；　are



equivalent。　Let　C　be　cloud；　A　thunder；　B　the　quenching　of　fire。　Then　B



is　attributable　to　C；　cloud；　since　fire　is　quenched　in　it；　and　A；



noise；　is　attributable　to　B；　and　B　is　assuredly　the　definition　of



the　major　term　A。　If　there　be　a　further　mediating　cause　of　B；　it



will　be　one　of　the　remaining　partial　definitions　of　A。



　　We　have　stated　then　how　essential　nature　is　discovered　and　becomes



known；　and　we　see　that；　while　there　is　no　syllogism…i。e。　no



demonstrative　syllogism…of　essential　nature；　yet　it　is　through



syllogism；　viz。　demonstrative　syllogism；　that　essential　nature　is



exhibited。　So　we　conclude　that　neither　can　the　essential　nature　of



anything　which　has　a　cause　distinct　from　itself　be　known　without



demonstration；　nor　can　it　be　demonstrated；　and　this　is　what　we



contended　in　our　preliminary　discussions。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　9







　　Now　while　some　things　have　a　cause　distinct　from　themselves；



others　have　not。　Hence　it　is　evident　that　there　are　essential



natures　which　are　immediate；　that　is　are　basic　premisses；　and　of　these



not　only　that　they　are　but　also　what　they　are　must　be　assumed　or



revealed　in　some　other　way。　This　too　is　the　actual　procedure　of　the



arithmetician；　who　assumes　both　the　nature　and　the　existence　of



unit。　On　the　other　hand；　it　is　possible　（in　the　manner　explained）　to



exhibit　through　demonstration　the　essential　nature　of　things　which



have　a　'middle'；　i。e。　a　cause　of　their　substantial　being　other　than



that　being　itself；　but　we　do　not　thereby　demonstrate　it。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　10







　　Since　definition　is　said　to　be　the　statement　of　a　thing's　nature；



obviously　one　kind　of　definition　will　be　a　statement　of　the　meaning　of



the　name；　or　of　an　equivalent　nominal　formula。　A　definition　in　this



sense　tells　you；　e。g。　the　meaning　of　the　phrase　'triangular



character'。　When　we　are　aware　that　triangle　exists；　we　inquire　the



reason　why　it　exists。　But　it　is　difficult　thus　to　learn　the　definition



of　things　the　existence　of　which　we　do　not　genuinely　know…the　cause　of



this　difficulty　being；　as　we　said　before；　that　we　only　know



accidentally　whether　or　not　the　thing　exists。　Moreover；　a　statement



may　be　a　unity　in　either　of　two　ways；　by　conjunction；　like　the



Iliad；　or　because　it　exhibits　a　single　predicate　as　inhering　not



accidentally　in　a　single　subject。



　　That　then　is　one　way　of　defining　definition。　Another　kind　of



definition　is　a　formula　exhibiting　the　cause　of　a　thing's　existence。



Thus　the　former　signifies　without　proving；　but　the　latter　will　clearly



be　a　quasi…demonstration　of　essential　nature；　differing　from



demonstration　in　the　arrangement　of　its　terms。　For　there　is　a



difference　between　stating　why　it　thunders；　and　stating　what　is　the



essential　nature　of　thunder；　since　the　first　statement　will　be



'Because　fire　is　quenched　in　the　clouds'；　while　the　statement　of



what　the　nature　of　thunder　is　will　be　'The　noise　of　fire　being



quenched　in　the　clouds'。　Thus　the　same　statement　takes　a　different



form：　in　one　form　it　is　continuous　demonstration；　in　the　other



definition。　Again；　thunder　can　be　defined　as　noise　in　the　clouds；



which　is　the　conclusion　of　the　demonstration　embodying　essential



nature。　On　the　other　hand　the　definition　of　immediates　is　an



indemonstrable　positing　of　essential　nature。



　　We　conclude　then　that　definition　is　（a）　an　indemonstrable



statement　of　essential　nature；　or　（b）　a　syllogism　of　essential



nature　differing　from　demonstration　in　grammatical　form；　or　（c）　the



conclusion　of　a　demonstration　giving　essential　nature。



　　Our　discussion　has　therefore　made　plain　（1）　in　what　sense　and　of



what　things　the　essential　nature　is　demonstrable；　and　in　what　sense



and　of　what　things　it　is　not；　（2）　what　are　the　various　meanings　of　the



term　definition；　and　in　what　sense　and　of　what　things　it　proves　the



essential　nature；　and　in　what　sense　and　of　what　things　it　does　not；



（3）　what　is　the　relation　of　definition　to　demonstration；　and　how　far



the　same　thing　is　both　definable　and　demonstrable　and　how　far　it　is



not。







　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　11







　　We　think　we　have　scientific　knowledge　when　we　know　the　cause；　and



there　are　four　causes：　（1）　the　definable　form；　（2）　an　antecedent　which



necessitates　a　consequent；　（3）　the　efficient　cause；　（4）　the　final



cause。　Hence　each　of　these　can　be　the　middle　term　of　a　proof；　for



（a）　though　the　inference　from　antecedent　to　necessary　consequent



does　not　hold　if　only　one　premiss　is　assumed…two　is　the



minimum…still　when　there　are　two　it　holds　on　condition　that　they



have　a　single　common　middle　term。　So　it　is　from　the　assumption　of　this



single　middle　term　that　the　conclusion　follows　necessarily。　The



following　example　will　also　show　this。　Why　is　the　angle　in　a



semicircle　a　right　angle？…or　from　what　assumption　does　it　follow



that　it　is　a　right　angle？　Thus；　let　A　be　right　angle；　B　the　half　of



two　right　angles；　C　the　angle　in　a　semicircle。　Then　B　is　the　cause



in　virtue　of　which　A；　right　angle；　is　attributable　to　C；　the　angle



in　a　semicircle；　since　B=A　and　the　other；　viz。　C；=B；　for　C　is　half



of　two　right　angles。　Therefore　it　is　the　assumption　of　B；　the　half



of　two　right　angles；　from　which　it　follows　that　A　is　attributable　to



C；　i。e。　that　the　angle　in　a　semicircle　is　a　right　angle。　Moreover；　B



is　identical　with　（b）　the　defining　form　of　A；　since　it　is　what　A's



definition　signifies。　Moreover；　the　formal　cause　has　already　been



shown　to　be　the　middle。　（c）　'Why　did　the　Athenians　become　involved



in　the　Persian　war？'　means　'What　cause　originated　the　waging　of　war



against　the　Athenians？'　and　the　answer　is；　'Because　they　raided　Sardis



with　the　Eretrians'；　since　this　originated　the　war。　Let　A　be　war；　B



unprovoked　raiding；　C　the　Athenians。　Then　B；　unprovoked　raiding；　is



true　of　C；　the　Athenians；　and　A　is　true　of　B；　since　men　make　war　on



the　unjust　aggressor。　So　A；　having　war　waged　upon　them；　is　true　of



B；　the　initial　aggressors；　and　B　is　true　of　C；　the　Athenians；　who　were



the　aggressors。　Hence　here　too　the　cause…in　this　case　the　efficient



cause…is　the　middle　term。　（d）　This　is　no　less　true　where　th