a history of science-1-第21章

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s　this　persecution　takes　one　form；　sometimes　another；　to　the　credit　of　the　Greeks　be　it　said；　that　with　them　it　usually　led　to　nothing　more　severe　than　banishment。　In　the　case　of　Anaxagoras；　it　is　alleged　that　the　sentence　pronounced　was　death；　but　that；　thanks　to　the　influence　of　Pericles；　this　sentence　was　commuted　to　banishment。　In　any　event；　the　aged　philosopher　was　sent　away　from　the　city　of　his　adoption。　He　retired　to　Lampsacus。　〃It　is　not　I　that　have　lost　the　Athenians；〃　he　said；　〃it　is　the　Athenians　that　have　lost　me。〃　The　exact　position　which　Anaxagoras　had　among　his　contemporaries；　and　his　exact　place　in　the　development　of　philosophy；　have　always　been　somewhat　in　dispute。　It　is　not　known；　of　a　certainty；　that　he　even　held　an　open　school　at　Athens。　Ritter　thinks　it　doubtful　that　he　did。　It　was　his　fate　to　be　misunderstood；　or　underestimated；　by　Aristotle；　that　in　itself　would　have　sufficed　greatly　to　dim　his　famemight；　indeed；　have　led　to　his　almost　entire　neglect　had　he　not　been　a　truly　remarkable　thinker。　With　most　of　the　questions　that　have　exercised　the　commentators　we　have　but　scant　concern。　Following　Aristotle；　most　historians　of　philosophy　have　been　metaphysicians；　they　have　concerned　themselves　far　less　with　what　the　ancient　thinkers　really　knew　than　with　what　they　thought。　A　chance　using　of　a　verbal　quibble；　an　esoteric　phrase；　the　expression　of　a　vague　mysticismthese　would　suffice　to　call　forth　reams　of　exposition。　It　has　been　the　favorite　pastime　of　historians　to　weave　their　own　anachronistic　theories　upon　the　scanty　woof　of　the　half…　remembered　thoughts　of　the　ancient　philosophers。　To　make　such　cloth　of　the　imagination　as　this　is　an　alluring　pastime；　but　one　that　must　not　divert　us　here。　Our　point　of　view　reverses　that　of　the　philosophers。　We　are　chiefly　concerned；　not　with　some　vague　saying　of　Anaxagoras；　but　with　what　he　really　knew　regarding　the　phenomena　of　nature；　with　what　he　observed；　and　with　the　comprehensible　deductions　that　he　derived　from　his　observations。　In　attempting　to　answer　these　inquiries；　we　are　obliged；　in　part；　to　take　our　evidence　at　second…hand；　but；　fortunately；　some　fragments　of　writings　of　Anaxagoras　have　come　down　to　us。　We　are　told　that　he　wrote　only　a　single　book。　It　was　said　even　（by　Diogenes）　that　he　was　the　first　man　that　ever　wrote　a　work　in　prose。　The　latter　statement　would　not　bear　too　close　an　examination；　yet　it　is　true　that　no　extensive　prose　compositions　of　an　earlier　day　than　this　have　been　preserved；　though　numerous　others　are　known　by　their　fragments。　Herodotus；　〃the　father　of　prose；〃　was　a　slightly　younger　contemporary　of　the　Clazomenaean　philosopher；　not　unlikely　the　two　men　may　have　met　at　Athens。　Notwithstanding　the　loss　of　the　greater　part　of　the　writings　of　Anaxagoras；　however；　a　tolerably　precise　account　of　his　scientific　doctrines　is　accessible。　Diogenes　Laertius　expresses　some　of　them　in　very　clear　and　precise　terms。　We　have　already　pointed　out　the　uncertainty　that　attaches　to　such　evidence　as　this；　but　it　is　as　valid　for　Anaxagoras　as　for　another。　If　we　reject　such　evidence；　we　shall　often　have　almost　nothing　left；　in　accepting　it　we　may　at　least　feel　certain　that　we　are　viewing　the　thinker　as　his　contemporaries　and　immediate　successors　viewed　him。　Following　Diogenes；　then；　we　shall　find　some　remarkable　scientific　opinions　ascribed　to　Anaxagoras。　〃He　asserted；〃　we　are　told；　〃that　the　sun　was　a　mass　of　burning　iron；　greater　than　Peloponnesus；　and　that　the　moon　contained　houses　and　also　hills　and　ravines。〃　In　corroboration　of　this；　Plato　represents　him　as　having　conjectured　the　right　explanation　of　the　moon's　light；　and　of　the　solar　and　lunar　eclipses。　He　had　other　astronomical　theories　that　were　more　fanciful；　thus　〃he　said　that　the　stars　originally　moved　about　in　irregular　confusion；　so　that　at　first　the　pole…star；　which　is　continually　visible；　always　appeared　in　the　zenith；　but　that　afterwards　it　acquired　a　certain　declination；　and　that　the　Milky　Way　was　a　reflection　of　the　light　of　the　sun　when　the　stars　did　not　appear。　The　comets　he　considered　to　be　a　concourse　of　planets　emitting　rays；　and　the　shooting…　stars　he　thought　were　sparks；　as　it　were；　leaping　from　the　firmament。〃　Much　of　this　is　far　enough　from　the　truth；　as　we　now　know　it；　yet　all　of　it　shows　an　earnest　endeavor　to　explain　the　observed　phenomena　of　the　heavens　on　rational　principles。　To　have　predicated　the　sun　as　a　great　molten　mass　of　iron　was　indeed　a　wonderful　anticipation　of　the　results　of　the　modern　spectroscope。　Nor　can　it　be　said　that　this　hypothesis　of　Anaxagoras　was　a　purely　visionary　guess。　It　was　in　all　probability　a　scientific　deduction　from　the　observed　character　of　meteoric　stones。　Reference　has　already　been　made　to　the　alleged　prediction　of　the　fall　of　the　famous　meteor　at　aegespotomi　by　Anaxagoras。　The　assertion　that　he　actually　predicted　this　fall　in　any　proper　sense　of　the　word　would　be　obviously　absurd。　Yet　the　fact　that　his　name　is　associated　with　it　suggests　that　he　had　studied　similar　meteorites；　or　else　that　he　studied　this　particular　one；　since　it　is　not　quite　clear　whether　it　was　before　or　after　this　fall　that　he　made　the　famous　assertion　that　space　is　full　of　falling　stones。　We　should　stretch　the　probabilities　were　we　to　assert　that　Anaxagoras　knew　that　shooting…stars　and　meteors　were　the　same；　yet　there　is　an　interesting　suggestiveness　in　his　likening　the　shooting…stars　to　sparks　leaping　from　the　firmament；　taken　in　connection　with　his　observation　on　meteorites。　Be　this　as　it　may；　the　fact　that　something　which　falls　from　heaven　as　a　blazing　light　turns　out　to　be　an　iron…like　mass　may　very　well　have　suggested　to　the　most　rational　of　thinkers　that　the　great　blazing　light　called　the　sun　has　the　same　composition。　This　idea　grasped；　it　was　a　not　unnatural　extension　to　conceive　the　other　heavenly　bodies　as　having　the　same　composition。　This　led　to　a　truly　startling　thought。　Since　the　heavenly　bodies　are　of　the　same　composition　as　the　earth；　and　since　they　are　observed　to　be　whirling　about　the　earth　in　space；　may　we　not　suppose　that　they　were　once　a　part　of　the　earth　itself；　and　that　they　have　been　thrown　off　by　the　force　of　a　whirling　motion？　Such　was　the　conclusion　which　Anaxagoras　reached；　such　his　explanation　of　the　origin　of　the　heavenly　bodies。　It　was　a　marvellous　guess。　Deduct　from　it　all　that　recent　science　has　shown　to　be　untrue；　bear　in　mind　that　the　stars　are　suns；　compared　with　which　the　earth　is　a　mere　speck　of　dust；　recall　that　the　sun　is　parent；　not　daughter；　of　the　earth；　and　despite　all　these　deductions；　the　cosmogonic　guess　of　Anaxagoras　remains；　as　it　seems　to　us；　one　of　the　most　marvellous　feats　of　human　intelligence。　It　was　the　first　explanation　of　the　cosmic　bodies　that　could　be　called；　in　any　sense；　an　anticipation　of　what　the　science　of　our　own　day　accepts　as　a　true　explanation　of　cosmic　origins。　Moreover；　let　us　urge　again　that　this　was　no　mere　accidental　flight　of　the　imagination；　it　was　a　scientific　induction　based　on　the　only　data　available；　perhaps　it　is　not　too　much　to　say　that　it　was　the　only　scientific　induction　which　these　data　would　fairly　sustain。　Of　course　it　is　not　for　a　moment　to　be　inferred　that　Anaxagoras　understood；　in　the　modern　sense；　the　character　of　that　whirling　force　which　we　call　centrifugal。　About　two　thousand　years　were　yet　to　elapse　before　that　force　was　explained　as　elementary　inertia；　and　even　that　explanation；　let　us　not　forget；　merely　sufficed　to　push　back　the　barriers　of　mystery　by　one　other　stage；　for　even　in　our　day　inertia　is　a　statement　of　fact　rather　than　an　explanation。　But　however　little　Anaxagoras　could　explain　the　centrifugal　force　on　mechanical　principles；　the　practical　powers　of　that　force　were　sufficiently　open　to　his　observation。　The　mere　experiment　of　throwing　a　stone　from　a　sling　would；　to　an　observing　mind；　be　full　of　suggestiveness。　It　would　be　obvious　that　by　whirling　the　sling　about；　the　stone　which　it　held　would　be　sustained　in　its　circling　path　about　the　hand　in　seeming　defiance　of　the　earth's　pull；　and　after　the　stone　had　left　the　sling；　it　could　fly　away　from　the　earth　to　a　distance　which　the　most　casual　observation　would　prove　to　be　proportionate　to　the　speed　of　its　flight。　Extremely　rapid　motion；　then；　might　project　bodies　from　the　earth's　surface　off　into　space；　a　sufficiently　rapid　whirl　would　keep　them　there。　Anaxagoras　conceived　that　this　was　precisely　what　had　occurred。　His　imagination　even　carried　him　a　step　fartherto　a　conception　of　a　slackening　of　speed；　through　which　the　heavenly　bodies　would　lose　their　centrifugal　force；　and；　responding　to　the　perpetual　pull　of　gravitation；　would　fall　back　to　the　earth；　just　as　the　great　stone　at　aegespotomi　had　been　observed　to　do。　Here　we　would　seem　to　have　a　clear　conception　of　the　idea　of　universal　gravitation；　and　Anaxagoras　stands　before　us　as　the　anticipator　of　Newton。　Were　it　not　for　one　scientific　maxim；　we　might　exalt　the　old　Greek　above　the　greatest　of　modern　natural　philosophers；　but　that　maxim　bids　us　pause。　It　is　phrased　thus；　〃He　discovers　who　proves。〃　Anaxagoras　could　not　prove；　his　argument　was　at　best　suggestive；　not　demonstrative。　He　did　not　even　know　the　laws　which　govern　falling　bodies；　much　less　could　he　apply　such　laws；　even　had　he　known　them；　to　sidereal　bodies　at　whose　size　and　distance　he　could　only　guess　in　the　vaguest　terms。　Still　his　cosmogonic　speculation　remains　as　perhaps　the　most　remarkable　one　of　antiquity。　How　widely　his　speculation　found　currency　among　his　immediate　successors　is　instanced　in　a　passage　from　Plato；　where　Socrates　is　represented　as　scornfully　answering　a　calumniator　in　these　terms：　〃He　asserts　that　I　say　the　sun　is　a　stone　and　the　moon　an　earth。　Do　you　think　of　accusing　Anaxagoras；　Miletas；　and　have　you　so　low　an　opinion　of　these　men；　and　think　them　so　unskilled　in　laws；　as　not　to　know　that　the　books　of　Anaxagoras　the　Clazomenaean　are　full　of　these　doctrines。　And　forsooth　the　young　men　are　learning　these　matters　from　me　which　sometimes　they　can　buy　from　the　orchestra　for　a　drachma；　at　the　most；　and　laugh　at　Socrates　if　he　pretends　they　are　his…particularly　seeing　they　are　so　strange。〃　The　element　of　error　contained　in　these　cosmogonic　speculations　of　Anaxagoras　has　led　critics　to　do　them　something　less　than　justice。　But　there　is　one　other　astronomical　speculation　for　which　the　Clazomenaean　philosopher　has　received　full　credit。　It　is　generally　admitted　that　it　was　he　who　first　found　out　the　explanation　of　the　phases　of　the　moon；　a　knowledge　that　that　body　shines　only　by　reflected　light；　and　that　its　visible　forms；　waxing　and　waning　month　by　month　from　crescent　to　disk　and　from　disk　to　crescent；　merely　represent　our　shifting　view　of　its　sun…illumined　face。　It　is　difficult　to　put　ourselves　in　the　place　of　the　ancient　observer　and　realize　how　little　the　appearances　suggest　the　actual　fact。　That　a　body　of　the　same　structure　as　the　earth　should　shine　with　the　radiance　of　the　moon　merely　because　sunlight　is　reflected　from　it；　is　in　itself　a　supposition　seemingly　contradicted　by　ordinary　experience。　It　required　the　mind　of　a　philosopher；　sustained；　perhaps；　by　some　experimental　observations；　to　conceive　the　idea　that　what　seems　so　obviously　bright　may　be　in　reality　dark。　The　germ　of　the　conception　of　what　the　philosopher　spea