Thereisnoconnectionwhateverbetweenthesoundoftheword’man’andthe’ideas’whichthewordexcites，beyondthefactthatthesoundhasbeenpreviouslyheardwhentheideaswereexcited。Here，then，isaphenomenontobeexplainedorgeneralised。Wehaveincountlesscasesacertainconnectionestablishedforwhichnofurtherreasoncanbeassignedthanthefactofitspreviousoccurrence。Onsuchaground，webelievethatfireburns，thatbreadiswholesome，thatstonesfall；andbutforsuchbeliefscouldknownothingoftheoutsideworld。’Contingent’truth，therefore，ortruthderivedfrommerecontact。pervades，ifitdoesnotconstitute，thewholefabricofourwholeknowledge。Toprovethatallourknowledgeisderivedfromexperienceis，accordingtoMill，toprovethatinsomesenseorotherassociationofideasliesatthebaseofallintellectualprocesses。WhenLockeintroducedachapterupon’AssociationofIdeas’intothefourtheditionofhisessay，hetreateditastheexceptionalcase。Someideashadaconnectiontraceablebyreason；otherswereonlyconnectedby’chanceandcustom。’Associationdoesnotexplainreasoning，onlythedeviationsfromreasoning。ButwithHumeandHartleytherelationisinverted。Theprinciple，insteadofbeinganexceptionalcase，issimplytheuniversalrulefromwhichlogicalconnectionmaybededucedasaspecialcase。
ThefactsuponwhichMillrelied，andtheaccountofthemwhichhegave，requirenoticeandembodimentinanyroundpsychology。Insomeshapeorothertheyformthestarting-pointofalllatersystems。Mill’svigorousapplicationofhisprinciple，workedoutwithimperfectappreciationandwithmanyoversights，hadtherefrom，atleast，themeritofpreparingthegroundforamorescientificmethod。
Inanycase，however，hisconclusions，sofarassound，mustbeplacedinadifferentframeworkoftheory。Itbecomesnecessarytodwellchieflyuponthecuriousdefectsofhistheory，iftakenashewishedittobetaken，foranultimatescientificstatement。Thefactthatthereisasynthesisandananalysisisexpressedby’association。’Butwhatmorecanwesay?
Whatarethe’laws’ofassociation?Unlesssomerulecanbegiven，weshallgetnothingthatcanbecalledatheory。Oneideaisnotsuggestedbytheotherthroughanylogicalprocess。Theyarestill’conjoined’butnot’connected。’
Theconnection，therefore，mustbegivenbysomethingdifferentfromtheideasthemselves。Nowtheorderoftheoriginal’sensations’dependsuponthe’objectsofnature，’andisthereforeleftto’physicalphilosophy。’53Theyoccur，however，eitherin’synchronous’orin’successive’order。