PASSAGEONE(1)Attheageoftwelveyears,thehumanbodyisatitsmostvigorous.Ithasyettoreachitsfullsizeandstrength,anditsownerhisorherfullintelligence;butatthisagethelikelihoodofdeathisleast.Earlier,wewereinfantsandyoungchildren,andconsequentlymorevulnerable;later,weshallundergoaprogressivelossofourvigorandresistancewhich,thoughimperceptibleatfirst,willfinallybecomesosteepthatwecanlivenolonger,howeverwellwelookafterourselves,andhoweverwellsociety,andourdoctors,lookafterus.(2)Thisdeclineinvigorwiththepassingoftimeiscalledageing.Itisoneofthemostunpleasantdiscoverieswhichweallmakethatwemustdeclineinthisway,thatifweescapewars,accidentsanddiseaseweshalleventually"dieofoldage",andthatthishappensataratewhichdifferslittlefrompersontoperson,sothatthereareheavyoddsinfavorofourdyingbetweentheagesofsixty-fiveandeighty.Someofuswilldiesooner,afewwilllivelonger—onintoaninthortenthdecade.Butthechancesareagainstit,andthereisavirtuallimitonhowlongwecanhopetoremainalive,howeverluckyandrobustweare.(3)Normalpeopletendtoforgetthisprocessunlessanduntiltheyareremindedofit.Wearesofamiliarwiththefactthatmanages,thatpeoplehaveforyearsassumedthattheprocessoflosingvigorwithtime,ofbecomingmorelikelytodietheolderweget,wassomethingself-evident,likethecoolingofahotkettleorthewearing-outofapairofshoes.Theyhavealsoassumedthatallanimals,andprobablyotherorganismssuchastrees,oreventheuniverseitself,mustinthenatureofthings"wearout".(4)Mostanimalswecommonlyobservedoinfactageaswedo,ifgiventhechancetolivelongenough;andmechanicalsystemslikeawoundwatch,orthesun,doinfactanoutofenergyinaccordancewiththesecondlawofthermodynamics(热力学)(whetherthewholeuniversedoessoisamootpointatpresent).Butthesearenotanalogoustowhathappenswhenmanages.Arun-downwatchisstillawatchandcanberewound.Anoldwatch,bycontrast,becomessowornandunreliablethatiteventuallyisnotworthmending.Butawatchcouldneverrepairitself—itdoesnotconsistoflivingparts,onlyofmetal,whichwearsawaybyfriction.Wecould,atonetime,repairourselves—wellenough,atleast,toovercomeallbutthemostinstantlyfatalillnessesandaccidents.Betweentwelveandeightyyearswegraduallylosethispower;anillnesswhichattwelvewouldknockusover,ateightycanknockusout,andintoourgrave.Ifwecouldstayasvigorousasweareattwelve,itwouldtakeabout700yearsforhalfofustodie,andanother700forthesurvivorstobereducedbyhalfagain.22.Theword"it"inthelastsentenceofParagraphTworefersto()