of　the　several　senses。
  12
  The　following　results　applying　to　any　and　every　sense　may　now　be　formulated。　　　（A）　By　a　'sense'　is　meant　what　has　the　power　of　receiving　into　itself　the　sensible　forms　of　things　without　the　matter。　This　must　be　conceived　of　as　taking　place　in　the　way　in　which　a　piece　of　wax　takes　on　the　impress　of　a　signet…ring　without　the　iron　or　gold；　we　say　that　what　produces　the　impression　is　a　signet　of　bronze　or　gold；　but　its　particular　metallic　constitution　makes　no　difference：　in　a　similar　way　the　sense　is　affected　by　what　is　coloured　or　flavoured　or　sounding；　but　it　is　indifferent　what　in　each　case　the　substance　is；　what　alone　matters　is　what　quality　it　has；　i。e。　in　what　ratio　its　constituents　are　combined。　　　（B）　By　'an　organ　of　sense'　is　meant　that　in　which　ultimately　such　a　power　is　seated。　　　The　sense　and　its　organ　are　the　same　in　fact；　but　their　essence　is　not　the　same。　What　perceives　is；　of　course；　a　spatial　magnitude；　but　we　must　not　admit　that　either　the　having　the　power　to　perceive　or　the　sense　itself　is　a　magnitude；　what　they　are　is　a　certain　ratio　or　power　in　a　magnitude。　This　enables　us　to　explain　why　objects　of　sense　which　possess　one　of　two　opposite　sensible　qualities　in　a　degree　largely　in　excess　of　the　other　opposite　destroy　the　organs　of　sense；　if　the　movement　set　up　by　an　object　is　too　strong　for　the　organ；　the　equipoise　of　contrary　qualities　in　the　organ；　which　just　is　its　sensory　power；　is　disturbed；　it　is　precisely　as　concord　and　tone　are　destroyed　by　too　violently　twanging　the　strings　of　a　lyre。　This　explains　also　why　plants　cannot　perceive。　in　spite　of　their　having　a　portion　of　soul　in　them　and　obviously　being　affected　by　tangible　objects　themselves；　for　undoubtedly　their　temperature　can　be　lowered　or　raised。　The　explanation　is　that　they　have　no　mean　of　contrary　qualities；　and　so　no　principle　in　them　capable　of　taking　on　the　forms　of　sensible　objects　without　their　matter；　in　the　case　of　plants　the　affection　is　an　affection　by　form…and…matter　together。　The　problem　might　be　raised：　Can　what　cannot　smell　be　said　to　be　affected　by　smells　or　what　cannot　see　by　colours；　and　so　on？　It　might　be　said　that　a　smell　is　just　what　can　be　smelt；　and　if　it　produces　any　effect　it　can　only　be　so　as　to　make　something　smell　it；　and　it　might　be　argued　that　what　cannot　smell　cannot　be　affected　by　smells　and　further　that　what　can　smell　can　be　affected　by　it　only　in　so　far　as　it　has　in　it　the　power　to　smell　（similarly　with　the　proper　objects　of　all　the　other　senses）。　Indeed　that　this　is　so　is　made　quite　evident　as　follows。　Light　or　darkness；　sounds　and　smells　leave　bodies　quite　unaffected；　what　does　affect　bodies　is　not　these　but　the　bodies　which　are　their　vehicles；　e。g。　what　splits　the　trunk　of　a　tree　is　not　the　sound　of　the　thunder　but　the　air　which　accompanies　thunder。　Yes；　but；　it　may　be　objected；　bodies　are　affected　by　what　is　tangible　and　by　flavours。　If　not；　by　what　are　things　that　are　without　soul　affected；　i。e。　altered　in　quality？　Must　we　not；　then；　admit　that　the　objects　of　the　other　senses　also　may　affect　them？　Is　not　the　true　account　this；　that　all　bodies　are　capable　of　being　affected　by　smells　and　sounds；　but　that　some　on　being　acted　upon；　having　no　boundaries　of　their　own；　disintegrate；　as　in　the　instance　of　air；　which　does　become　odorous；　showing　that　some　effect　is　produced　on　it　by　what　is　odorous？　But　smelling　is　more　than　such　an　affection　by　what　is　odorous…what　more？　Is　not　the　answer　that；　while　the　air　owing　to　the　momentary　duration　of　the　action　upon　it　of　what　is　odorous　does　itself　become　perceptible　to　the　sense　of　smell；　smelling　is　an　observing　of　the　result　produced？
  Book　III　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1
  THAT　there　is　no　sixth　sense　in　addition　to　the　five　enumerated…sight；　hearing；　smell；　taste；　touch…may　be　established　by　the　following　considerations：　　　If　we　have　actually　sensation　of　everything　of　which　touch　can　give　us　sensation　（for　all　the　qualities　of　the　tangible　qua　tangible　are　perceived　by　us　through　touch）；　and　if　absence　of　a　sense　necessarily　involves　absence　of　a　sense…organ；　and　if　（1）　all　objects　that　we　perceive　by　immediate　contact　with　them　are　perceptible　by　touch；　which　sense　we　actually　possess；　and　（2）　all　objects　that　we　perceive　through　media；　i。e。　without　immediate　contact；　are　perceptible　by　or　through　the　simple　elements；　e。g。　air　and　water　（and　this　is　so　arranged　that　（a）　if　more　than　one　kind　of　sensible　object　is　perceivable　through　a　single　medium；　the　possessor　of　a　sense…organ　homogeneous　with　that　medium　has　the　power　of　perceiving　both　kinds　of　objects；　for　example；　if　the　sense…organ　is　made　of　air；　and　air　is　a　medium　both　for　sound　and　for　colour；　and　that　（b）　if　more　than　one　medium　can　transmit　the　same　kind　of　sensible　objects；　as　e。g。　water　as　well　as　air　can　transmit　colour；　both　being　transparent；　then　the　possessor　of　either　alone　will　be　able　to　perceive　the　kind　of　objects　transmissible　through　both）；　and　if　of　the　simple　elements　two　only；　air　and　water；　go　to　form　sense…organs　（for　the　pupil　is　made　of　water；　the　organ　of　hearing　is　made　of　air；　and　the　organ　of　smell　of　one　or　other　of　these　two；　while　fire　is　found　either　in　none　or　in　all…warmth　being　an　essential　condition　of　all　sensibility…and　earth　either　in　none　or；　if　anywhere；　specially　mingled　with　the　components　of　the　organ　of　touch；　wherefore　it　would　remain　that　there　can　be　no　sense…organ　formed　of　anything　except　water　and　air）；　and　if　these　sense…organs　are　actually　found　in　certain　animals；…then　all　the　possible　senses　are　possessed　by　those　animals　that　are　not　imperfect　or　mutilated　（for　even　the　mole　is　observed　to　have　eyes　beneath　its　skin）；　so　that；　if　there　is　no　fifth　element　and　no　property　other　than　those　which　belong　to　the　four　elements　of　our　world；　no　sense　can　be　wanting　to　such　animals。　　　Further；　there　cannot　be　a　special　sense…organ　for　the　common　sensibles　either；　i。e。　the　objects　which　we　perceive　incidentally　through　this　or　that　special　sense；　e。g。　movement；　rest；　figure；　magnitude；　number；　unity；　for　all　these　we　perceive　by　movement；　e。g。　magnitude　by　movement；　and　therefore　also　figure　（for　figure　is　a　species　of　magnitude）；　what　is　at　rest　by　the　absence　of　movement：　number　is　perceived　by　the　negation　of　continuity；　and　by　the　special　sensibles；　for　each　sense　perceives　one　class　of　sensible　objects。　So　that　it　is　clearly　impossible　that　there　should　be　a　special　sense　for　any　one　of　the　common　sensibles；　e。g。　movement；　for；　if　that　were　so；　our　perception　of　it　would　be　exactly　parallel　to　our　present　perception　of　what　is　sweet　by　vision。　That　is　so　because　we　have　a　sense　for　each　of　the　two　qualities；　in　virtue　of　which　when　they　happen　to　meet　in　one　sensible　object　we　are　aware　of　both　contemporaneously。　If　it　were　not　like　this　our　perception　of　the　common　qualities　would　always　be　incidental；　i。e。　as　is　the　perception　of　Cleon's　son；　where　we　perceive　him　not　as　Cleon's　son　but　as　white；　and　the　white　thing　which　we　really　perceive　happens　to　be　Cleon's　son。　　　But　in　the　case　of　the　common　sensibles　there　is　already　in　us　a　general　sensibility　which　enables　us　to　perceive　them　directly；　there　is　therefore　no　special　sense　required　for　their　perception：　if　there　were；　our　perception　of　them　would　have　been　exactly　like　what　has　been　above　described。　　　The　senses　perceive　each　other's　special　objects　incidentally；　not　because　the　percipient　sense　is　this　or　that　special　sense；　but　because　all　form　a　unity：　this　incidental　perception　takes　place　whenever　sense　is　directed　at　one　and　the　same　moment　to　two　disparate　qualities　in　one　and　the　same　object；　e。g。　to　the　bitterness　and　the　yellowness　of　bile；　the　assertion　of　the　identity　of　both　cannot　be　the　act　of　either　of　the　senses；　hence　the　illusion　of　sense；　e。g。　the　belief　that　if　a　thing　is　yellow　it　is　bile。　　　It　might　be　asked　why　we　have　more　senses　than　one。　Is　it　to　prevent　a　failure　to　apprehend　the　common　sensibles；　e。g。　movement；　magnitude；　and　number；　which　go　along　with　the　special　sensibles？　Had　we　no　sense　but　sight；　and　that　sense　no　object　but　white；　they　would　have　tended　to　escape　our　notice　and　everything　would　have　merged　for　us　into　an　indistinguishable　identity　because　of　the　concomitance　of　colour　and　magnitude。　As　it　is；　the　fact　that　the　common　sensibles　are　given　in　the　objects　of　more　than　one　sense　reveals　their　distinction　from　each　and　all　of　the　special　sensibles。
  2
  Since　it　is　through　sense　that　we　are　aware　that　we　are　seeing　or　hearing；　it　must　be　either　by　sight　that　we　are　aware　of　seeing；　or　by　some　sense　other　than　sight。　But　the　sense　that　gives　us　this　new　sensation　must　perceive　both　sight　and　its　object；　viz。　colour：　so　that　either　（1）　there　will　be　two　senses　both　percipient　of　the　same　sensible　object；　or　（2）　the　sense　must　be　percipient　of　itself。　Further；　even　if　the　sense　which　perceives　sight　were　different　from　sight；　we　must　either　fall　into　an　infinite　regress；　or　we　must　somewhere　assume　a　sense　which　is　aware　of　itself。　If　so；　we　ought　to　do　this　in　the　first　case。　　　This　presents　a　difficulty：　if　to　perceive　by