posterior analytics-第3章

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demonstrations；　it　is　clearly　frivolous　and　impossible　to　say　that



demonstration　is　reciprocal　and　that　therefore　everything　can　be



demonstrated。







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　　Since　the　object　of　pure　scientific　knowledge　cannot　be　other　than



it　is；　the　truth　obtained　by　demonstrative　knowledge　will　be



necessary。　And　since　demonstrative　knowledge　is　only　present　when　we



have　a　demonstration；　it　follows　that　demonstration　is　an　inference



from　necessary　premisses。　So　we　must　consider　what　are　the　premisses



of　demonstration…i。e。　what　is　their　character：　and　as　a　preliminary；



let　us　define　what　we　mean　by　an　attribute　'true　in　every　instance



of　its　subject'；　an　'essential'　attribute；　and　a　'commensurate　and



universal'　attribute。　I　call　'true　in　every　instance'　what　is　truly



predicable　of　all　instances…not　of　one　to　the　exclusion　of



others…and　at　all　times；　not　at　this　or　that　time　only；　e。g。　if　animal



is　truly　predicable　of　every　instance　of　man；　then　if　it　be　true　to



say　'this　is　a　man'；　'this　is　an　animal'　is　also　true；　and　if　the



one　be　true　now　the　other　is　true　now。　A　corresponding　account　holds



if　point　is　in　every　instance　predicable　as　contained　in　line。　There



is　evidence　for　this　in　the　fact　that　the　objection　we　raise　against　a



proposition　put　to　us　as　true　in　every　instance　is　either　an



instance　in　which；　or　an　occasion　on　which；　it　is　not　true。



Essential　attributes　are　（1）　such　as　belong　to　their　subject　as



elements　in　its　essential　nature　（e。g。　line　thus　belongs　to



triangle；　point　to　line；　for　the　very　being　or　'substance'　of　triangle



and　line　is　composed　of　these　elements；　which　are　contained　in　the



formulae　defining　triangle　and　line）：　（2）　such　that；　while　they　belong



to　certain　subjects；　the　subjects　to　which　they　belong　are　contained



in　the　attribute's　own　defining　formula。　Thus　straight　and　curved



belong　to　line；　odd　and　even；　prime　and　compound；　square　and　oblong；



to　number；　and　also　the　formula　defining　any　one　of　these　attributes



contains　its　subject…e。g。　line　or　number　as　the　case　may　be。



　　Extending　this　classification　to　all　other　attributes；　I　distinguish



those　that　answer　the　above　description　as　belonging　essentially　to



their　respective　subjects；　whereas　attributes　related　in　neither　of



these　two　ways　to　their　subjects　I　call　accidents　or　'coincidents'；



e。g。　musical　or　white　is　a　'coincident'　of　animal。



　　Further　（a）　that　is　essential　which　is　not　predicated　of　a　subject



other　than　itself：　e。g。　'the　walking　＇thing＇'　walks　and　is　white　in



virtue　of　being　something　else　besides；　whereas　substance；　in　the



sense　of　whatever　signifies　a　'this　somewhat'；　is　not　what　it　is　in



virtue　of　being　something　else　besides。　Things；　then；　not　predicated



of　a　subject　I　call　essential；　things　predicated　of　a　subject　I　call



accidental　or　'coincidental'。



　　In　another　sense　again　（b）　a　thing　consequentially　connected　with



anything　is　essential；　one　not　so　connected　is　'coincidental'。　An



example　of　the　latter　is　'While　he　was　walking　it　lightened'：　the



lightning　was　not　due　to　his　walking；　it　was；　we　should　say；　a



coincidence。　If；　on　the　other　hand；　there　is　a　consequential



connexion；　the　predication　is　essential；　e。g。　if　a　beast　dies　when　its



throat　is　being　cut；　then　its　death　is　also　essentially　connected　with



the　cutting；　because　the　cutting　was　the　cause　of　death；　not　death　a



'coincident'　of　the　cutting。



　　So　far　then　as　concerns　the　sphere　of　connexions　scientifically



known　in　the　unqualified　sense　of　that　term；　all　attributes　which



（within　that　sphere）　are　essential　either　in　the　sense　that　their



subjects　are　contained　in　them；　or　in　the　sense　that　they　are



contained　in　their　subjects；　are　necessary　as　well　as



consequentially　connected　with　their　subjects。　For　it　is　impossible



for　them　not　to　inhere　in　their　subjects　either　simply　or　in　the



qualified　sense　that　one　or　other　of　a　pair　of　opposites　must　inhere



in　the　subject；　e。g。　in　line　must　be　either　straightness　or　curvature；



in　number　either　oddness　or　evenness。　For　within　a　single　identical



genus　the　contrary　of　a　given　attribute　is　either　its　privative　or　its



contradictory；　e。g。　within　number　what　is　not　odd　is　even；　inasmuch　as



within　this　sphere　even　is　a　necessary　consequent　of　not…odd。　So；



since　any　given　predicate　must　be　either　affirmed　or　denied　of　any



subject；　essential　attributes　must　inhere　in　their　subjects　of



necessity。



　　Thus；　then；　we　have　established　the　distinction　between　the



attribute　which　is　'true　in　every　instance'　and　the　'essential'



attribute。



　　I　term　'commensurately　universal'　an　attribute　which　belongs　to



every　instance　of　its　subject；　and　to　every　instance　essentially　and



as　such；　from　which　it　clearly　follows　that　all　commensurate



universals　inhere　necessarily　in　their　subjects。　The　essential



attribute；　and　the　attribute　that　belongs　to　its　subject　as　such；



are　identical。　E。g。　point　and　straight　belong　to　line　essentially；　for



they　belong　to　line　as　such；　and　triangle　as　such　has　two　right



angles；　for　it　is　essentially　equal　to　two　right　angles。



　　An　attribute　belongs　commensurately　and　universally　to　a　subject



when　it　can　be　shown　to　belong　to　any　random　instance　of　that



subject　and　when　the　subject　is　the　first　thing　to　which　it　can　be



shown　to　belong。　Thus；　e。g。　（1）　the　equality　of　its　angles　to　two



right　angles　is　not　a　commensurately　universal　attribute　of　figure。



For　though　it　is　possible　to　show　that　a　figure　has　its　angles　equal



to　two　right　angles；　this　attribute　cannot　be　demonstrated　of　any



figure　selected　at　haphazard；　nor　in　demonstrating　does　one　take　a



figure　at　random…a　square　is　a　figure　but　its　angles　are　not　equal



to　two　right　angles。　On　the　other　hand；　any　isosceles　triangle　has　its



angles　equal　to　two　right　angles；　yet　isosceles　triangle　is　not　the



primary　subject　of　this　attribute　but　triangle　is　prior。　So　whatever



can　be　shown　to　have　its　angles　equal　to　two　right　angles；　or　to



possess　any　other　attribute；　in　any　random　instance　of　itself　and



primarily…that　is　the　first　subject　to　which　the　predicate　in　question



belongs　commensurately　and　universally；　and　the　demonstration；　in



the　essential　sense；　of　any　predicate　is　the　proof　of　it　as



belonging　to　this　first　subject　commensurately　and　universally：



while　the　proof　of　it　as　belonging　to　the　other　subjects　to　which　it



attaches　is　demonstration　only　in　a　secondary　and　unessential　sense。



Nor　again　（2）　is　equality　to　two　right　angles　a　commensurately



universal　attribute　of　isosceles；　it　is　of　wider　application。







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　　We　must　not　fail　to　observe　that　we　often　fall　into　error　because



our　conclusion　is　not　in　fact　primary　and　commensurately　universal



in　the　sense　in　which　we　think　we　prove　it　so。　We　make　this　mistake



（1）　when　the　subject　is　an　individual　or　individuals　above　which　there



is　no　universal　to　be　found：　（2）　when　the　subjects　belong　to　different



species　and　there　is　a　higher　universal；　but　it　has　no　name：　（3）



when　the　subject　which　the　demonstrator　takes　as　a　whole　is　really



only　a　part　of　a　larger　whole；　for　then　the　demonstration　will　be　true



of　the　individual　instances　within　the　part　and　will　hold　in　every



instance　of　it；　yet　the　demonstration　will　not　be　true　of　this　subject



primarily　and　commensurately　and　universally。　When　a　demonstration



is　true　of　a　subject　primarily　and　commensurately　and　universally；



that　is　to　be　taken　to　mean　that　it　is　true　of　a　given　subject



primarily　and　as　such。　Cas