Therealdifficultyinformulatingthegeneraldefinitionofsupplyanddemandisthatitseemstotakeontheappearanceofatautology。Firstconsiderthesupply——theproductavailableinthemarket，orthatwhichcanbedeliveredtoit。toavoiddwellinguponuselessdetail，weshallhereconsideronlythemassannuallyreproducedineverygivenbranchofproductionandignorethegreaterorlesserfacultypossessedbythedifferentcommoditiestobewithdrawnfromthemarketandstoredawayforconsumption，say，untilnextyear。Thisannualreproductionisexpressedbyacertainquantity——inweightornumbers——dependingonwhetherthismassofcommoditiesismeasuredindiscreteelementsorcontinuously。Theyarenotonlyuse-valuessatisfyinghumanwants，buttheseuse-valuesareavailableinthemarketindefinitequantities。Secondly，however，thisquantityofcommoditieshasaspecificmarket-value，whichmaybeexpressedbyamultipleofthemarket-valueofthecommodity，orofitsmeasure，whichservesasunit。
Thus，thereisnonecessaryconnectionbetweenthequantitativevolumeofthecommoditiesinthemarketandtheirmarket-value，since，forinstance，manycommoditieshaveaspecificallyhighvalue，andothersaspecificallylowvalue，sothatagivensumofvaluesmayberepresentedbyaverylargequantityofonecommodity，andaverysmallquantityofanother。Thereisonlythefollowingconnectionbetweenthequantityofthearticlesavailableinthemarketandthemarket-valueofthesearticles:Onagivenbasisoflabourproductivitytheproductionofacertainquantityofarticlesineveryparticularsphereofproductionrequiresadefinitequantityofsociallabour-time；althoughthisproportionvariesindifferentspheresofproductionandhasnoinnerrelationtotheusefulnessofthesearticlesorthespecialnatureoftheiruse-values。Assumingallothercircumstancestobeequal，andacertainquantityaofsomecommoditytocostblabour-time，aquantitynaofthesamecommoditywillcostnblabour-time。Further，ifsocietywantstosatisfysomewantandhaveanarticleproducedforthispurpose，itmustpayforit。Indeed，sincecommodity-productionnecessitatesadivisionoflabour，societypaysforthisarticlebydevotingaportionoftheavailablelabour-timetoitsproduction。Therefore，societybuysitwithadefinitequantityofitsdisposablelabour-time。Thatpartofsocietywhichthroughthedivisionoflabourhappenstoemployitslabourinproducingthisparticulararticle，mustreceiveanequivalentinsociallabourincorporatedinarticleswhichsatisfyitsownwants。However，thereexistsanaccidentalratherthananecessaryconnectionbetweenthetotalamountofsociallabourappliedtoasocialarticle，i。e。，betweenthealiquotpartofsociety’stotallabour-powerallocatedtoproducingthisarticle，orbetweenthevolumewhichtheproductionofthisarticleoccupiesintotalproduction，ontheonehand，andthevolumewherebysocietyseekstosatisfythewantgratifiedbythearticleinquestion，ontheother。
Everyindividualarticle，oreverydefinitequantityofacommoditymay，indeed，containnomorethanthesociallabourrequiredforitsproduction，andfromthispointofviewthemarket-valueofthisentirecommodityrepresentsonlynecessarylabour，butifthiscommodityhasbeenproducedinexcessoftheexistingsocialneeds，thensomuchofthesociallabour-timeissquanderedandthemassofthecommoditycomestorepresentamuchsmallerquantityofsociallabourinthemarketthanisactuallyincorporatedinit。Itisonlywhereproductionisundertheactual，predeterminingcontrolofsocietythatthelatterestablishesarelationbetweenthevolumeofsociallabour-timeappliedinproducingdefinitearticles，andthevolumeofthesocialwanttobesatisfiedbythesearticles。Forthisreason，thesecommoditiesmustbesoldbelowtheirmarket-value，andaportionofthemmayevenbealtogetherunsaleable。Thereverseappliesifthequantityofsociallabouremployedintheproductionofacertainkindofcommodityistoosmalltomeetthesocialdemandforthatcommodity。Butifthequantityofsociallabourexpendedintheproductionofacertainarticlecorrespondstothesocialdemandforthatarticle，sothattheproducedquantitycorrespondstotheusualscaleofreproductionandthedemandremainsunchanged，thenthearticleissoldatitsmarket-value。Theexchange，orsale，ofcommoditiesattheirvalueistherationalstateofaffairs，i。e。，thenaturallawoftheirequilibrium。Itisthislawthatexplainsthedeviations，andnotviceversa，thedeviationsthatexplainthelaw。
Nowletuslookattheotherside——thedemand。
Commoditiesareboughteitherasmeansofproductionormeansofsubsistencetoenterproductiveorindividualconsumption。Itdoesnotaltermattersthatsomecommoditiesmayservebothpurposes。Thereis，then，ademandforthemonthepartofproducersherecapitalists，sincewehaveassumedthatmeansofproductionhavebeentransformedintocapitalandofconsumers。