▁of ▁value ", ▁but ▁this ▁is ▁strictly ▁speaking ▁an ▁error , ▁for ▁five ▁reasons . ▁The ▁law ▁of ▁value ▁only ▁states ▁a ▁general ▁regul ative ▁principle ▁about ▁the ▁necessary ▁and ▁in ev itable ▁relationship ▁between ▁the ▁trad ing ▁values ▁of ▁comm od ities , ▁and ▁the ▁soci ally ▁average ▁labour - time ▁required ▁to ▁supply ▁them . ▁It ▁is ▁simply ▁a ▁law ▁gover ning ▁comm od ity ▁exchange . ▁The ▁labour ▁ ▁of ▁value ▁in ▁econom ics ▁a ims ▁to ▁explain ▁ ▁that ▁determ ination ▁actually ▁works , ▁what ▁kinds ▁of ▁caus al ▁relationships ▁are ▁involved , ▁how ▁the ▁law ▁of ▁value ▁interact s ▁with ▁other ▁economic ▁laws , ▁etc . ▁For ▁Marx ▁himself , ▁the ▁" lab our ▁theory ▁of ▁value " ▁referred ▁only ▁to ▁the ▁theory ▁of ▁value ▁u ph eld ▁by ▁some ▁of ▁the ▁classical ▁political ▁econom ists ▁from ▁William ▁Pet ty ▁to ▁David ▁Ric ardo , ▁who ▁regarded ▁human ▁labour ▁as ▁the ▁real ▁subst ance ▁of ▁product ▁value . ▁ ▁Marx ' s ▁own ▁value ▁theory ▁is ▁not ▁a ▁theory ▁of ▁ ▁value , ▁but ▁only ▁of ▁the ▁value - system ▁involved ▁in ▁comm od ity ▁production ▁and ▁comm od ity ▁trade . ▁ ▁Marx ▁never ▁referred ▁to ▁his ▁own ▁theory ▁as ▁a ▁" lab our ▁theory ▁of ▁value "; ▁his ▁own ▁critique ▁of ▁the ▁political ▁econom ists ▁was , ▁that ▁they ▁all ▁failed ▁to ▁explain ▁satisf actor ily ▁ ▁the ▁determ ination ▁of ▁product - value ▁by ▁labour - time ▁actually ▁worked — they ▁assumed ▁it , ▁but