▁differential ▁and ▁absolute ▁ground ▁rent . ▁World ▁market ▁prices ▁for ▁primary ▁products ▁can ▁at ▁any ▁time ▁be ▁strongly ▁influenced ▁by ▁the ▁yield ▁of ▁har v ests ▁and ▁mines ▁in ▁different ▁countries , ▁regardless ▁of ▁labour ▁effort . ▁There ▁are , ▁besides , ▁various ▁kinds ▁of ▁products ▁which , ▁for ▁one ▁reason ▁or ▁another , ▁are ▁not ▁subject ▁to ▁the ▁law ▁of ▁value ▁( see ▁below ). ▁ ▁Orig ins ▁of ▁the ▁concept ▁According ▁to ▁Marx , ▁the ▁knowledge ▁that ▁the ▁law ▁of ▁value ▁existed , ▁expressed ▁in ▁one ▁form ▁or ▁another , ▁sometimes ▁more ▁clearly ▁and ▁sometimes ▁less , ▁was ▁very ▁ancient — it ▁reached ▁right ▁back ▁to ▁the ▁first ▁nom adic ▁trad ers ▁in ▁food , ▁craft s , ▁services ▁and ▁min er als . ▁People ▁knew ▁very ▁well ▁that ▁there ▁was ▁a ▁definite ▁relationship ▁between ▁time ▁worked ▁and ▁the ▁value ▁of ▁products ▁trad ed ; ▁in ▁itself ▁that ▁was ▁not ▁a ▁very ▁difficult ▁insight ▁to ▁grasp . ▁In ▁fact , ▁three ▁hundred ▁years ▁before ▁the ▁Scottish ▁and ▁English ▁political ▁econom ists , ▁I bn ▁Kh ald un ▁had ▁already ▁formally ▁presented ▁a ▁fairly ▁s oph istic ated ▁understanding ▁of ▁the ▁law ▁of ▁value . ▁The ▁economic ▁effects ▁of ▁the ▁avail ability ▁or ▁lack ▁of ▁labour — al ready ▁reck oned ▁with ▁some ▁precision ▁in ▁ancient ▁S umer ▁more ▁than ▁four ▁thousand ▁years ▁ago — w ere ▁rather ▁self - ev ident ▁in ▁practical ▁life . ▁Nevertheless , ▁different ▁think ers ▁in ▁history ▁failed ▁to ▁concept ual ize ▁the ▁law ▁of ▁value