velopment　of　an　existent　quality　from　potentiality　in　the　direction　of　fixity　or　nature。　　　In　the　case　of　what　is　to　possess　sense；　the　first　transition　is　due　to　the　action　of　the　male　parent　and　takes　place　before　birth　so　that　at　birth　the　living　thing　is；　in　respect　of　sensation；　at　the　stage　which　corresponds　to　the　possession　of　knowledge。　Actual　sensation　corresponds　to　the　stage　of　the　exercise　of　knowledge。　But　between　the　two　cases　compared　there　is　a　difference；　the　objects　that　excite　the　sensory　powers　to　activity；　the　seen；　the　heard；　&c。；　are　outside。　The　ground　of　this　difference　is　that　what　actual　sensation　apprehends　is　individuals；　while　what　knowledge　apprehends　is　universals；　and　these　are　in　a　sense　within　the　soul。　That　is　why　a　man　can　exercise　his　knowledge　when　he　wishes；　but　his　sensation　does　not　depend　upon　himself　a　sensible　object　must　be　there。　A　similar　statement　must　be　made　about　our　knowledge　of　what　is　sensible…on　the　same　ground；　viz。　that　the　sensible　objects　are　individual　and　external。　　　A　later　more　appropriate　occasion　may　be　found　thoroughly　to　clear　up　all　this。　At　present　it　must　be　enough　to　recognize　the　distinctions　already　drawn；　a　thing　may　be　said　to　be　potential　in　either　of　two　senses；　（a）　in　the　sense　in　which　we　might　say　of　a　boy　that　he　may　become　a　general　or　（b）　in　the　sense　in　which　we　might　say　the　same　of　an　adult；　and　there　are　two　corresponding　senses　of　the　term　'a　potential　sentient'。　There　are　no　separate　names　for　the　two　stages　of　potentiality；　we　have　pointed　out　that　they　are　different　and　how　they　are　different。　We　cannot　help　using　the　incorrect　terms　'being　acted　upon　or　altered'　of　the　two　transitions　involved。　As　we　have　said；　has　the　power　of　sensation　is　potentially　like　what　the　perceived　object　is　actually；　that　is；　while　at　the　beginning　of　the　process　of　its　being　acted　upon　the　two　interacting　factors　are　dissimilar；　at　the　end　the　one　acted　upon　is　assimilated　to　the　other　and　is　identical　in　quality　with　it。
  6
  In　dealing　with　each　of　the　senses　we　shall　have　first　to　speak　of　the　objects　which　are　perceptible　by　each。　The　term　'object　of　sense'　covers　three　kinds　of　objects；　two　kinds　of　which　are；　in　our　language；　directly　perceptible；　while　the　remaining　one　is　only　incidentally　perceptible。　Of　the　first　two　kinds　one　（a）　consists　of　what　is　perceptible　by　a　single　sense；　the　other　（b）　of　what　is　perceptible　by　any　and　all　of　the　senses。　I　call　by　the　name　of　special　object　of　this　or　that　sense　that　which　cannot　be　perceived　by　any　other　sense　than　that　one　and　in　respect　of　which　no　error　is　possible；　in　this　sense　colour　is　the　special　object　of　sight；　sound　of　hearing；　flavour　of　taste。　Touch；　indeed；　discriminates　more　than　one　set　of　different　qualities。　Each　sense　has　one　kind　of　object　which　it　discerns；　and　never　errs　in　reporting　that　what　is　before　it　is　colour　or　sound　（though　it　may　err　as　to　what　it　is　that　is　coloured　or　where　that　is；　or　what　it　is　that　is　sounding　or　where　that　is。）　Such　objects　are　what　we　propose　to　call　the　special　objects　of　this　or　that　sense。　　　'Common　sensibles'　are　movement；　rest；　number；　figure；　magnitude；　these　are　not　peculiar　to　any　one　sense；　but　are　common　to　all。　There　are　at　any　rate　certain　kinds　of　movement　which　are　perceptible　both　by　touch　and　by　sight。　　　We　speak　of　an　incidental　object　of　sense　where　e。g。　the　white　object　which　we　see　is　the　son　of　Diares；　here　because　'being　the　son　of　Diares'　is　incidental　to　the　directly　visible　white　patch　we　speak　of　the　son　of　Diares　as　being　（incidentally）　perceived　or　seen　by　us。　Because　this　is　only　incidentally　an　object　of　sense；　it　in　no　way　as　such　affects　the　senses。　Of　the　two　former　kinds；　both　of　which　are　in　their　own　nature　perceptible　by　sense；　the　first　kind…that　of　special　objects　of　the　several　senses…constitute　the　objects　of　sense　in　the　strictest　sense　of　the　term　and　it　is　to　them　that　in　the　nature　of　things　the　structure　of　each　several　sense　is　adapted。
  7
  The　object　of　sight　is　the　visible；　and　what　is　visible　is　（a）　colour　and　（b）　a　certain　kind　of　object　which　can　be　described　in　words　but　which　has　no　single　name；　what　we　mean　by　（b）　will　be　abundantly　clear　as　we　proceed。　Whatever　is　visible　is　colour　and　colour　is　what　lies　upon　what　is　in　its　own　nature　visible；　'in　its　own　nature'　here　means　not　that　visibility　is　involved　in　the　definition　of　what　thus　underlies　colour；　but　that　that　substratum　contains　in　itself　the　cause　of　visibility。　Every　colour　has　in　it　the　power　to　set　in　movement　what　is　actually　transparent；　that　power　constitutes　its　very　nature。　That　is　why　it　is　not　visible　except　with　the　help　of　light；　it　is　only　in　light　that　the　colour　of　a　thing　is　seen。　Hence　our　first　task　is　to　explain　what　light　is。　　　Now　there　clearly　is　something　which　is　transparent；　and　by　'transparent'　I　mean　what　is　visible；　and　yet　not　visible　in　itself；　but　rather　owing　its　visibility　to　the　colour　of　something　else；　of　this　character　are　air；　water；　and　many　solid　bodies。　Neither　air　nor　water　is　transparent　because　it　is　air　or　water；　they　are　transparent　because　each　of　them　has　contained　in　it　a　certain　substance　which　is　the　same　in　both　and　is　also　found　in　the　eternal　body　which　constitutes　the　uppermost　shell　of　the　physical　Cosmos。　Of　this　substance　light　is　the　activity…the　activity　of　what　is　transparent　so　far　forth　as　it　has　in　it　the　determinate　power　of　becoming　transparent；　where　this　power　is　present；　there　is　also　the　potentiality　of　the　contrary；　viz。　darkness。　Light　is　as　it　were　the　proper　colour　of　what　is　transparent；　and　exists　whenever　the　potentially　transparent　is　excited　to　actuality　by　the　influence　of　fire　or　something　resembling　'the　uppermost　body'；　for　fire　too　contains　something　which　is　one　and　the　same　with　the　substance　in　question。　　　We　have　now　explained　what　the　transparent　is　and　what　light　is；　light　is　neither　fire　nor　any　kind　whatsoever　of　body　nor　an　efflux　from　any　kind　of　body　（if　it　were；　it　would　again　itself　be　a　kind　of　body）…it　is　the　presence　of　fire　or　something　resembling　fire　in　what　is　transparent。　It　is　certainly　not　a　body；　for　two　bodies　cannot　be　present　in　the　same　place。　The　opposite　of　light　is　darkness；　darkness　is　the　absence　from　what　is　transparent　of　the　corresponding　positive　state　above　characterized；　clearly　therefore；　light　is　just　the　presence　of　that。　　　Empedocles　（and　with　him　all　others　who　used　the　same　forms　of　expression）　was　wrong　in　speaking　of　light　as　'travelling'　or　being　at　a　given　moment　between　the　earth　and　its　envelope；　its　movement　being　unobservable　by　us；　that　view　is　contrary　both　to　the　clear　evidence　of　argument　and　to　the　observed　facts；　if　the　distance　traversed　were　short；　the　movement　might　have　been　unobservable；　but　where　the　distance　is　from　extreme　East　to　extreme　West；　the　draught　upon　our　powers　of　belief　is　too　great。　　　What　is　capable　of　taking　on　colour　is　what　in　itself　is　colourless；　as　what　can　take　on　sound　is　what　is　soundless；　what　is　colourless　includes　（a）　what　is　transparent　and　（b）　what　is　invisible　or　scarcely　visible；　i。e。　what　is　'dark'。　The　latter　（b）　is　the　same　as　what　is　transparent；　when　it　is　potentially；　not　of　course　when　it　is　actually　transparent；　it　is　the　same　substance　which　is　now　darkness；　now　light。　　　Not　everything　that　is　visible　depends　upon　light　for　its　visibility。　This　is　only　true　of　the　'proper'　colour　of　things。　Some　objects　of　sight　which　in　light　are　invisible；　in　darkness　stimulate　the　sense；　that　is；　things　that　appear　fiery　or　shining。　This　class　of　objects　has　no　simple　common　name；　but　instances　of　it　are　fungi；　flesh；　heads；　scales；　and　eyes　of　fish。　In　none　of　these　is　what　is　seen　their　own　proper'　colour。　Why　we　see　these　at　all　is　another　question。　At　present　what　is　obvious　is　that　what　is　seen　in　light　is　always　colour。　That　is　why　without　the　help　of　light　colour　remains　invisible。　Its　being　colour　at　all　means　precisely　its　having　in　it　the　power　to　set　in　movement　what　is　already　actually　transparent；　and；　as　we　have　seen；　the　actuality　of　what　is　transparent　is　just　light。　　　The　following　experiment　makes　the　necessity　of　a　medium　clear。　If　what　has　colour　is　placed　in　immediate　contact　with　the　eye；　it　cannot　be　seen。　Colour　sets　in　movement　not　the　sense　organ　but　what　is　transparent；　e。g。　the　air；　and　that；　extending　continuously　from　the　object　to　the　organ；　sets　the　latter　in　movement。　Democritus　misrepresents　the　facts　when　he　expresses　the　opinion　that　if　the　interspace　were　empty　one　could　distinctly　see　an　ant　on　the　vault　of　the　sky；　that　is　an　impossibility。　Seeing　is　due　to　an　affection　or　change　of　what　has　the　perceptive　faculty；　and　it　cannot　be　affected　by　the　seen　colour　itself；　it　remains　that　it　must　be　affected　by　what　comes　between。　Hence　it　is　indispensable　that　there　be　something　in　between…if　there　were　nothing；　so　far　from　seeing　with　greater　distinctness；　we　should　see　nothing　at　all。　　　We　have　now　explained　the　cause　why　colour　cannot　be　seen　otherwise　than　in　light。　Fire　on　the　other　hand　is　seen　both　in　darkness　and　in　light；　this　double　possibility　follows