rse　of　nature　there　is　a　limit　or　ratio　which　determines　their　size　and　increase；　and　limit　and　ratio　are　marks　of　soul　but　not　of　fire；　and　belong　to　the　side　of　formulable　essence　rather　than　that　of　matter。　　　Nutrition　and　reproduction　are　due　to　one　and　the　same　psychic　power。　It　is　necessary　first　to　give　precision　to　our　account　of　food；　for　it　is　by　this　function　of　absorbing　food　that　this　psychic　power　is　distinguished　from　all　the　others。　The　current　view　is　that　what　serves　as　food　to　a　living　thing　is　what　is　contrary　to　it…not　that　in　every　pair　of　contraries　each　is　food　to　the　other：　to　be　food　a　contrary　must　not　only　be　transformable　into　the　other　and　vice　versa；　it　must　also　in　so　doing　increase　the　bulk　of　the　other。　Many　a　contrary　is　transformed　into　its　other　and　vice　versa；　where　neither　is　even　a　quantum　and　so　cannot　increase　in　bulk；　e。g。　an　invalid　into　a　healthy　subject。　It　is　clear　that　not　even　those　contraries　which　satisfy　both　the　conditions　mentioned　above　are　food　to　one　another　in　precisely　the　same　sense；　water　may　be　said　to　feed　fire；　but　not　fire　water。　Where　the　members　of　the　pair　are　elementary　bodies　only　one　of　the　contraries；　it　would　appear；　can　be　said　to　feed　the　other。　But　there　is　a　difficulty　here。　One　set　of　thinkers　assert　that　like　fed；　as　well　as　increased　in　amount；　by　like。　Another　set；　as　we　have　said；　maintain　the　very　reverse；　viz。　that　what　feeds　and　what　is　fed　are　contrary　to　one　another；　like；　they　argue；　is　incapable　of　being　affected　by　like；　but　food　is　changed　in　the　process　of　digestion；　and　change　is　always　to　what　is　opposite　or　to　what　is　intermediate。　Further；　food　is　acted　upon　by　what　is　nourished　by　it；　not　the　other　way　round；　as　timber　is　worked　by　a　carpenter　and　not　conversely；　there　is　a　change　in　the　carpenter　but　it　is　merely　a　change　from　not…working　to　working。　In　answering　this　problem　it　makes　all　the　difference　whether　we　mean　by　'the　food'　the　'finished'　or　the　'raw'　product。　If　we　use　the　word　food　of　both；　viz。　of　the　completely　undigested　and　the　completely　digested　matter；　we　can　justify　both　the　rival　accounts　of　it；　taking　food　in　the　sense　of　undigested　matter；　it　is　the　contrary　of　what　is　fed　by　it；　taking　it　as　digested　it　is　like　what　is　fed　by　it。　Consequently　it　is　clear　that　in　a　certain　sense　we　may　say　that　both　parties　are　right；　both　wrong。　　　Since　nothing　except　what　is　alive　can　be　fed；　what　is　fed　is　the　besouled　body　and　just　because　it　has　soul　in　it。　Hence　food　is　essentially　related　to　what　has　soul　in　it。　Food　has　a　power　which　is　other　than　the　power　to　increase　the　bulk　of　what　is　fed　by　it；　so　far　forth　as　what　has　soul　in　it　is　a　quantum；　food　may　increase　its　quantity；　but　it　is　only　so　far　as　what　has　soul　in　it　is　a　'this…somewhat'　or　substance　that　food　acts　as　food；　in　that　case　it　maintains　the　being　of　what　is　fed；　and　that　continues　to　be　what　it　is　so　long　as　the　process　of　nutrition　continues。　Further；　it　is　the　agent　in　generation；　i。e。　not　the　generation　of　the　individual　fed　but　the　reproduction　of　another　like　it；　the　substance　of　the　individual　fed　is　already　in　existence；　the　existence　of　no　substance　is　a　self…generation　but　only　a　self…maintenance。　　　Hence　the　psychic　power　which　we　are　now　studying　may　be　described　as　that　which　tends　to　maintain　whatever　has　this　power　in　it　of　continuing　such　as　it　was；　and　food　helps　it　to　do　its　work。　That　is　why；　if　deprived　of　food；　it　must　cease　to　be。　　　The　process　of　nutrition　involves　three　factors；　（a）　what　is　fed；　（b）　that　wherewith　it　is　fed；　（c）　what　does　the　feeding；　of　these　（c）　is　the　first　soul；　（a）　the　body　which　has　that　soul　in　it；　（b）　the　food。　But　since　it　is　right　to　call　things　after　the　ends　they　realize；　and　the　end　of　this　soul　is　to　generate　another　being　like　that　in　which　it　is；　the　first　soul　ought　to　be　named　the　reproductive　soul。　The　expression　（b）　'wherewith　it　is　fed'　is　ambiguous　just　as　is　the　expression　'wherewith　the　ship　is　steered'；　that　may　mean　either　（i）　the　hand　or　（ii）　the　rudder；　i。e。　either　（i）　what　is　moved　and　sets　in　movement；　or　（ii）　what　is　merely　moved。　We　can　apply　this　analogy　here　if　we　recall　that　all　food　must　be　capable　of　being　digested；　and　that　what　produces　digestion　is　warmth；　that　is　why　everything　that　has　soul　in　it　possesses　warmth。　　　We　have　now　given　an　outline　account　of　the　nature　of　food；　further　details　must　be　given　in　the　appropriate　place。
  5
  Having　made　these　distinctions　let　us　now　speak　of　sensation　in　the　widest　sense。　Sensation　depends；　as　we　have　said；　on　a　process　of　movement　or　affection　from　without；　for　it　is　held　to　be　some　sort　of　change　of　quality。　Now　some　thinkers　assert　that　like　is　affected　only　by　like；　in　what　sense　this　is　possible　and　in　what　sense　impossible；　we　have　explained　in　our　general　discussion　of　acting　and　being　acted　upon。　　　Here　arises　a　problem：　why　do　we　not　perceive　the　senses　themselves　as　well　as　the　external　objects　of　sense；　or　why　without　the　stimulation　of　external　objects　do　they　not　produce　sensation；　seeing　that　they　contain　in　themselves　fire；　earth；　and　all　the　other　elements；　which　are　the　direct　or　indirect　objects　is　so　of　sense？　It　is　clear　that　what　is　sensitive　is　only　potentially；　not　actually。　The　power　of　sense　is　parallel　to　what　is　combustible；　for　that　never　ignites　itself　spontaneously；　but　requires　an　agent　which　has　the　power　of　starting　ignition；　otherwise　it　could　have　set　itself　on　fire；　and　would　not　have　needed　actual　fire　to　set　it　ablaze。　　　In　reply　we　must　recall　that　we　use　the　word　'perceive'　in　two　ways；　for　we　say　（a）　that　what　has　the　power　to　hear　or　see；　'sees'　or　'hears'；　even　though　it　is　at　the　moment　asleep；　and　also　（b）　that　what　is　actually　seeing　or　hearing；　'sees'　or　'hears'。　Hence　'sense'　too　must　have　two　meanings；　sense　potential；　and　sense　actual。　Similarly　'to　be　a　sentient'　means　either　（a）　to　have　a　certain　power　or　（b）　to　manifest　a　certain　activity。　To　begin　with；　for　a　time；　let　us　speak　as　if　there　were　no　difference　between　（i）　being　moved　or　affected；　and　（ii）　being　active；　for　movement　is　a　kind　of　activity…an　imperfect　kind；　as　has　elsewhere　been　explained。　Everything　that　is　acted　upon　or　moved　is　acted　upon　by　an　agent　which　is　actually　at　work。　Hence　it　is　that　in　one　sense；　as　has　already　been　stated；　what　acts　and　what　is　acted　upon　are　like；　in　another　unlike；　i。e。　prior　to　and　during　the　change　the　two　factors　are　unlike；　after　it　like。　　　But　we　must　now　distinguish　not　only　between　what　is　potential　and　what　is　actual　but　also　different　senses　in　which　things　can　be　said　to　be　potential　or　actual；　up　to　now　we　have　been　speaking　as　if　each　of　these　phrases　had　only　one　sense。　We　can　speak　of　something　as　'a　knower'　either　（a）　as　when　we　say　that　man　is　a　knower；　meaning　that　man　falls　within　the　class　of　beings　that　know　or　have　knowledge；　or　（b）　as　when　we　are　speaking　of　a　man　who　possesses　a　knowledge　of　grammar；　each　of　these　is　so　called　as　having　in　him　a　certain　potentiality；　but　there　is　a　difference　between　their　respective　potentialities；　the　one　（a）　being　a　potential　knower；　because　his　kind　or　matter　is　such　and　such；　the　other　（b）；　because　he　can　in　the　absence　of　any　external　counteracting　cause　realize　his　knowledge　in　actual　knowing　at　will。　This　implies　a　third　meaning　of　'a　knower'　（c）；　one　who　is　already　realizing　his　knowledge…he　is　a　knower　in　actuality　and　in　the　most　proper　sense　is　knowing；　e。g。　this　A。　Both　the　former　are　potential　knowers；　who　realize　their　respective　potentialities；　the　one　（a）　by　change　of　quality；　i。e。　repeated　transitions　from　one　state　to　its　opposite　under　instruction；　the　other　（b）　by　the　transition　from　the　inactive　possession　of　sense　or　grammar　to　their　active　exercise。　The　two　kinds　of　transition　are　distinct。　　　Also　the　expression　'to　be　acted　upon'　has　more　than　one　meaning；　it　may　mean　either　（a）　the　extinction　of　one　of　two　contraries　by　the　other；　or　（b）　the　maintenance　of　what　is　potential　by　the　agency　of　what　is　actual　and　already　like　what　is　acted　upon；　with　such　likeness　as　is　compatible　with　one's　being　actual　and　the　other　potential。　For　what　possesses　knowledge　becomes　an　actual　knower　by　a　transition　which　is　either　not　an　alteration　of　it　at　all　（being　in　reality　a　development　into　its　true　self　or　actuality）　or　at　least　an　alteration　in　a　quite　different　sense　from　the　usual　meaning。　　　Hence　it　is　wrong　to　speak　of　a　wise　man　as　being　'altered'　when　he　uses　his　wisdom；　just　as　it　would　be　absurd　to　speak　of　a　builder　as　being　altered　when　he　is　using　his　skill　in　building　a　house。　　　What　in　the　case　of　knowing　or　understanding　leads　from　potentiality　to　actuality　ought　not　to　be　called　teaching　but　something　else。　That　which　starting　with　the　power　to　know　learns　or　acquires　knowledge　through　the　agency　of　one　who　actually　knows　and　has　the　power　of　teaching　either　（a）　ought　not　to　be　said　'to　be　acted　upon'　at　all　or　（b）　we　must　recognize　two　senses　of　alteration；　viz。　（i）　the　substitution　of　one　quality　for　another；　the　first　being　the　contrary　of　the　second；　or　（ii）　the　development　of　an　existent　quality　from　potentiality　in　the　direction　of　fixity　or　nature。　　　In　the　ca