the　same　mode　of　being　as　a　line　or　a　plane。
  2
  For　our　study　of　soul　it　is　necessary；　while　formulating　the　problems　of　which　in　our　further　advance　we　are　to　find　the　solutions；　to　call　into　council　the　views　of　those　of　our　predecessors　who　have　declared　any　opinion　on　this　subject；　in　order　that　we　may　profit　by　whatever　is　sound　in　their　suggestions　and　avoid　their　errors。　　　The　starting…point　of　our　inquiry　is　an　exposition　of　those　characteristics　which　have　chiefly　been　held　to　belong　to　soul　in　its　very　nature。　Two　characteristic　marks　have　above　all　others　been　recognized　as　distinguishing　that　which　has　soul　in　it　from　that　which　has　not…movement　and　sensation。　It　may　be　said　that　these　two　are　what　our　predecessors　have　fixed　upon　as　characteristic　of　soul。　　　Some　say　that　what　originates　movement　is　both　pre…eminently　and　primarily　soul；　believing　that　what　is　not　itself　moved　cannot　originate　movement　in　another；　they　arrived　at　the　view　that　soul　belongs　to　the　class　of　things　in　movement。　This　is　what　led　Democritus　to　say　that　soul　is　a　sort　of　fire　or　hot　substance；　his　'forms'　or　atoms　are　infinite　in　number；　those　which　are　spherical　he　calls　fire　and　soul；　and　compares　them　to　the　motes　in　the　air　which　we　see　in　shafts　of　light　coming　through　windows；　the　mixture　of　seeds　of　all　sorts　he　calls　the　elements　of　the　whole　of　Nature　（Leucippus　gives　a　similar　account）；　the　spherical　atoms　are　identified　with　soul　because　atoms　of　that　shape　are　most　adapted　to　permeate　everywhere；　and　to　set　all　the　others　moving　by　being　themselves　in　movement。　This　implies　the　view　that　soul　is　identical　with　what　produces　movement　in　animals。　That　is　why；　further；　they　regard　respiration　as　the　characteristic　mark　of　life；　as　the　environment　compresses　the　bodies　of　animals；　and　tends　to　extrude　those　atoms　which　impart　movement　to　them；　because　they　themselves　are　never　at　rest；　there　must　be　a　reinforcement　of　these　by　similar　atoms　coming　in　from　without　in　the　act　of　respiration；　for　they　prevent　the　extrusion　of　those　which　are　already　within　by　counteracting　the　compressing　and　consolidating　force　of　the　environment；　and　animals　continue　to　live　only　so　long　as　they　are　able　to　maintain　this　resistance。　　　The　doctrine　of　the　Pythagoreans　seems　to　rest　upon　the　same　ideas；　some　of　them　declared　the　motes　in　air；　others　what　moved　them；　to　be　soul。　These　motes　were　referred　to　because　they　are　seen　always　in　movement；　even　in　a　complete　calm。　　　The　same　tendency　is　shown　by　those　who　define　soul　as　that　which　moves　itself；　all　seem　to　hold　the　view　that　movement　is　what　is　closest　to　the　nature　of　soul；　and　that　while　all　else　is　moved　by　soul；　it　alone　moves　itself。　This　belief　arises　from　their　never　seeing　anything　originating　movement　which　is　not　first　itself　moved。　　　Similarly　also　Anaxagoras　（and　whoever　agrees　with　him　in　saying　that　mind　set　the　whole　in　movement）　declares　the　moving　cause　of　things　to　be　soul。　His　position　must；　however；　be　distinguished　from　that　of　Democritus。　Democritus　roundly　identifies　soul　and　mind；　for　he　identifies　what　appears　with　what　is　true…that　is　why　he　commends　Homer　for　the　phrase　'Hector　lay　with　thought　distraught'；　he　does　not　employ　mind　as　a　special　faculty　dealing　with　truth；　but　identifies　soul　and　mind。　What　Anaxagoras　says　about　them　is　more　obscure；　in　many　places　he　tells　us　that　the　cause　of　beauty　and　order　is　mind；　elsewhere　that　it　is　soul；　it　is　found；　he　says；　in　all　animals；　great　and　small；　high　and　low；　but　mind　（in　the　sense　of　intelligence）　appears　not　to　belong　alike　to　all　animals；　and　indeed　not　even　to　all　human　beings。　　　All　those；　then；　who　had　special　regard　to　the　fact　that　what　has　soul　in　it　is　moved；　adopted　the　view　that　soul　is　to　be　identified　with　what　is　eminently　originative　of　movement。　All；　on　the　other　hand；　who　looked　to　the　fact　that　what　has　soul　in　it　knows　or　perceives　what　is；　identify　soul　with　the　principle　or　principles　of　Nature；　according　as　they　admit　several　such　principles　or　one　only。　Thus　Empedocles　declares　that　it　is　formed　out　of　all　his　elements；　each　of　them　also　being　soul；　his　words　are：
  For　'tis　by　Earth　we　see　Earth；　by　Water　Water；　　　By　Ether　Ether　divine；　by　Fire　destructive　Fire；　　　By　Love　Love；　and　Hate　by　cruel　Hate。
  In　the　same　way　Plato　in　the　Timaeus　fashions　soul　out　of　his　elements；　for　like；　he　holds；　is　known　by　like；　and　things　are　formed　out　of　the　principles　or　elements；　so　that　soul　must　be　so　too。　Similarly　also　in　his　lectures　'On　Philosophy'　it　was　set　forth　that　the　Animal…itself　is　compounded　of　the　Idea　itself　of　the　One　together　with　the　primary　length；　breadth；　and　depth；　everything　else；　the　objects　of　its　perception；　being　similarly　constituted。　Again　he　puts　his　view　in　yet　other　terms：　Mind　is　the　monad；　science　or　knowledge　the　dyad　（because　it　goes　undeviatingly　from　one　point　to　another）；　opinion　the　number　of　the　plane；　sensation　the　number　of　the　solid；　the　numbers　are　by　him　expressly　identified　with　the　Forms　themselves　or　principles；　and　are　formed　out　of　the　elements；　now　things　are　apprehended　either　by　mind　or　science　or　opinion　or　sensation；　and　these　same　numbers　are　the　Forms　of　things。　　　Some　thinkers；　accepting　both　premisses；　viz。　that　the　soul　is　both　originative　of　movement　and　cognitive；　have　compounded　it　of　both　and　declared　the　soul　to　be　a　self…moving　number。　　　As　to　the　nature　and　number　of　the　first　principles　opinions　differ。　The　difference　is　greatest　between　those　who　regard　them　as　corporeal　and　those　who　regard　them　as　incorporeal；　and　from　both　dissent　those　who　make　a　blend　and　draw　their　principles　from　both　sources。　The　number　of　principles　is　also　in　dispute；　some　admit　one　only；　others　assert　several。　There　is　a　consequent　diversity　in　their　several　accounts　of　soul；　they　assume；　naturally　enough；　that　what　is　in　its　own　nature　originative　of　movement　must　be　among　what　is　primordial。　That　has　led　some　to　regard　it　as　fire；　for　fire　is　the　subtlest　of　the　elements　and　nearest　to　incorporeality；　further；　in　the　most　primary　sense；　fire　both　is　moved　and　originates　movement　in　all　the　others。　　　Democritus　has　expressed　himself　more　ingeniously　than　the　rest　on　the　grounds　for　ascribing　each　of　these　two　characters　to　soul；　soul　and　mind　are；　he　says；　one　and　the　same　thing；　and　this　thing　must　be　one　of　the　primary　and　indivisible　bodies；　and　its　power　of　originating　movement　must　be　due　to　its　fineness　of　grain　and　the　shape　of　its　atoms；　he　says　that　of　all　the　shapes　the　spherical　is　the　most　mobile；　and　that　this　is　the　shape　of　the　particles　of　fire　and　mind。　　　Anaxagoras；　as　we　said　above；　seems　to　distinguish　between　soul　and　mind；　but　in　practice　he　treats　them　as　a　single　substance；　except　that　it　is　mind　that　he　specially　posits　as　the　principle　of　all　things；　at　any　rate　what　he　says　is　that　mind　alone　of　all　that　is　simple；　unmixed；　and　pure。　He　assigns　both　characteristics；　knowing　and　origination　of　movement；　to　the　same　principle；　when　he　says　that　it　was　mind　that　set　the　whole　in　movement。　　　Thales；　too；　to　judge　from　what　is　recorded　about　him；　seems　to　have　held　soul　to　be　a　motive　force；　since　he　said　that　the　magnet　has　a　soul　in　it　because　it　moves　the　iron。　　　Diogenes　（and　others）　held　the　soul　to　be　air　because　he　believed　air　to　be　finest　in　grain　and　a　first　principle；　therein　lay　the　grounds　of　the　soul's　powers　of　knowing　and　originating　movement。　As　the　primordial　principle　from　which　all　other　things　are　derived；　it　is　cognitive；　as　finest　in　grain；　it　has　the　power　to　originate　movement。　　　Heraclitus　too　says　that　the　first　principle…the　'warm　exhalation'　of　which；　according　to　him；　everything　else　is　composed…is　soul；　further；　that　this　exhalation　is　most　incorporeal　and　in　ceaseless　flux；　that　what　is　in　movement　requires　that　what　knows　it　should　be　in　movement；　and　that　all　that　is　has　its　being　essentially　in　movement　（herein　agreeing　with　the　majority）。　　　Alcmaeon　also　seems　to　have　held　a　similar　view　about　soul；　he　says　that　it　is　immortal　because　it　resembles　'the　immortals；'　and　that　this　immortality　belongs　to　it　in　virtue　of　its　ceaseless　movement；　for　all　the　'things　divine；'　moon；　sun；　the　planets；　and　the　whole　heavens；　are　in　perpetual　movement。　　　of　More　superficial　writers；　some；　e。g。　Hippo；　have　pronounced　it　to　be　water；　they　seem　to　have　argued　from　the　fact　that　the　seed　of　all　animals　is　fluid；　for　Hippo　tries　to　refute　those　who　say　that　the　soul　is　blood；　on　the　ground　that　the　seed；　which　is　the　primordial　soul；　is　not　blood。　　　Another　group　（Critias；　for　example）　did　hold　it　to　be　blood；　they　take　perception　to　be　the　most　characteristic　attribute　of　soul；　and　hold　that　perceptiveness　is　due　to　the　nature　of　blood。　　　Each　of　the　elements　has　thus　found　its　partisan；　except　earth…earth　has　found　no　supporter　unless　we　count　as　such　those　who　have　declared　soul　to　be；　or　to　be　compounded　of；　all　the　elements。　All；　then；　it　may　be　said；　characterize　the　soul　by　three　marks；　Movement；　Sensation；　Incorporeality；　and　each　of　these　is　traced　back　to　the　first　principles。　That　is　why　（with　one　exception）　all　those　who　define　the　soul　by　its　power　of　knowing　mak