Inparticulartheactivityimmediatelyprecedingthetestwaskeptasconstantincharacteraswaspossible。Sincethementalaswellasthephysicalconditionofmanissubjecttoanevidentperiodicityof24hours，itwastakenforgrantedthatlikeexperimentalconditionsareobtainableonlyatliketimesofday。However，inordertocarryoutmorethanonetestinagivenday，differentexperimentswereoccasionallycarriedontogetheratdifferenttimesofday。Whentoogreatchangesintheouterandinnerlifeoccurred，thetestswerediscontinuedforalengthoftime。Theirresumptionwasprecededbysomedaysofrenewedtrainingvaryingaccordingtothelengthoftheinterruption。Section14。SourcesofErrorTheguidingpointofviewintheselectionofmaterialandindeterminingtherulesforitsemploymentwas，asisevident，theattempttosimplifyasfaraspossible，andtokeepasconstantaspossible，theconditionsunderwhichtheactivitytobeobserved，thatofmemory，cameintoplay。
Naturallythebetteronesucceedsinthisattemptthemoredoeshewithdrawfromthecomplicatedandchangingconditionsunderwhichthisactivitytakesplaceinordinarylifeandunderwhichitisofimportancetous。
Butthatisnoobjectiontothemethod。Thefreelyfallingbodyandthefrictionlessmachine，etc。，withwhichphysicsdeals，arealsoonlyabstractionswhencomparedwiththeactualhappeningsinnaturewhichareofimporttous。Wecanalmostnowheregetadirectknowledgeofthecomplicatedandthereal，butmustgetattheminroundaboutwaysbysuccessivecombinationsofexperiences，eachofwhichisobtainedinartificial，experimentalcases，rarelyorneverfurnishedinthisformbynature。
Meanwhilethefactthattheconnectionwiththeactivityofmemoryinordinarylifeisforthemomentlostisoflessimportancethanthereverse，namely，thatthisconnectionwiththecomplicationsandfluctuationsoflifeisnecessarilystillatoocloseone。Thestruggletoattainthemostsimpleanduniformconditionspossibleatnumerouspointsnaturallyencountersobstaclesthatarerootedinthenatureofthecaseandwhichthwarttheattempt。Theunavoidabledissimilarityofthematerialandtheequallyunavoidableirregularityoftheexternalconditionshavealreadybeentouchedupon。Ipassnexttotwootherunsurmountable[sic]sourcesofdifficulty。
Bymeansofthesuccessiverepetitionstheseriesare，sotospeak，raisedtoeverhigherlevels。Thenaturalassumptionwouldbethatatthemomentwhentheycouldforthefirsttimebereproducedbyheartthelevelthusattainedwouldalwaysbethesame。Ifthiswerethecase，i。e。，ifthischaracteristicfirstreproductionwereeverywhereaninvariableobjectivesignofanequallyinvariablefixednessoftheseries，itwouldbeofrealvaluetous。This，however，isnotactuallythecase。Theinnerconditionsoftheseparateseriesatthemomentofthefirstpossiblereproductionarenotalwaysthesame，andthemostthatcanbeassumedisthatinthecaseofthesedifferentseriestheseconditionsalwaysoscillateaboutthesamedegreeofinnersurety。Thisisclearlyseenifthelearningandrepeatingoftheseriesiscontinuedafterthatfirstspontaneousreproductionoftheserieshasbeenattained。Asageneralthingthecapacityforvoluntaryreproductionpersistsafterithasoncebeenreached。Innumerouscases，however，itdisappearsimmediatelyafteritsfirstappearance，andisregainedonlyafterseveralfurtherrepetitions。Thisprovesthatthepredispositionformemorisingtheseries，irrespectiveoftheirdifferencesofalargersortaccordingtothetimeofday，totheobjectiveandsubjectiveconditions，etc。，issubjecttosmallvariationsofshortduration，whethertheybecalledoscillationsofattentionorsomethingelse。If，attheveryinstantwhenthematerialtobememorisedhasalmostreachedthedesireddegreeofsurety，achancemomentofespecialmentalclearnessoccurs，thentheseriesiscaughtonthewingasitwere，oftentothelearner’ssurprise;