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point:itisstraightlinesthatsuggesttousthenotionofspace.Weseeand
movealongstraightlines.Movingalongthestraightline,wemovetowards
anobjective.Wearenotalwayssureofthepossibilityofreachingit.Hence
straightlinesgiveustheinitialsenseofthepossiblenatureoftheinfinite.
Planesareyetanotherelement.Weseeinspaceatleastoneplane,theplane
weseemtobein.Theinitialstageofgeometrycodifiesournotionsrelatedto
ourstayinginthatplane.Spacenotionscamelater.Thenwebegintonotice
otherplanesaswell.
Themutualdispositionofpoints,straightlinesandplanesissubjectto
definiterules(suchassay,thattwodifferentstraightlinescanhaveatmost
onecommonpoint,thattheyadheretoplanes,andsoon).Thataretruthsthat
mustbeacceptedwithoutproof(whichdoesnotmeanonfaith).Suchtruths
arecalledpostulates.Itisarguablewhetherpostulatesarefactssoobvious
thatnaturethruststhembeforeoureyesandallweneeddoisnotethem,or
whethertheystatementsaretheresultofslowlygrowingknowledgethatis
finallyspelledout,knowledgeofwhichwedonotknowwhetheritisfinal
andbeyonddoubt.Theevolutionofgeometrytellsusthatwhatistrueisthe
latterratherthantheformer.
Itisalsoarguablewhethertheformationofgeometricpostulatesbelongsin
thedomainofmathematics,orphilosophy,theguideoflearning.Aristotlewas
believedthattheissuebelongstophilosophy.Thisstatementshouldbeinterpret-
edassayingthattheissueismetamathematical,i.e.liesbeyondmathematics.
Weattributethequalityofcontinuitytoplansandstraightlines.
Butstraightlinesarecontinuawiththeearneststructure.Apointdivides
straightlineintotwoparts,eachofwhichisagainacontinuum.Thisproperty
ofastraightlineenablesustoorderthesetofitspoints.Wesaythatastraight
lineisanorderedcontinuum.Wealsosaythatitisone-dimensional.Neither
aplanenorspacehavethisproperty.
Whatisspace?Whydoesitexceedourimaginationandwhymust
achildlearnaboutit?Whydoevenaccomplishedpainterslosetheirway
whendealingwithperspective,asubjectwhoseknowledgeisonlyafew
centuryold,andproduceeither“flat”paintingsor“space”paintingsthatare
frequentlyflawed?Whycan3tweexitfromspaceintoanextradimension
thewayweexitfromaplane?Isitbecauseofalimitationofoursenses
orisitbecauseofthenatureofspace?Whilethefirstoftheseviewsis
verypopularandopensthedoortoavarietyofspeculations,thethreedi-
mensionalityofspaceisaphysicalfact;nomathematicalpremisesupports
thenumber3.Kantlinkedthenumberofdimensionswiththeformofthe
lawofgravitation.Canitbethatcountingdimensionsisanecessityofour
thoughtprocesses?
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