, ▁or , ▁for ▁that ▁matter , ▁that ▁total ▁profit ▁equals ▁total ▁sur plus - value . ▁On ▁that ▁ground ▁alone , ▁many ▁critics ▁argue , ▁there ▁is ▁already ▁no ▁proof ▁that ▁there ▁is ▁any ▁necessary ▁quant itative ▁relationship ▁between ▁them ▁( Mar x ▁simply ▁assumes ▁that ▁relationship , ▁but ▁does ▁not ▁prove ▁it ). ▁If ▁that ▁is ▁so , ▁then , ▁the ▁critics ▁argue , ▁there ▁is ▁no ▁sense ▁in ▁which ▁the ▁Marx ian ▁product - values ▁can ▁explain ▁market ▁prices ▁for ▁products ▁as ▁the ▁determin ants ▁of ▁those ▁prices . ▁An ▁additional ▁problem ▁discovered ▁in ▁mathematical ▁mod elling ▁is ▁that ▁the ▁assumption ▁of ▁the ▁identity ▁of ▁total ▁production ▁prices ▁and ▁total ▁values ▁( or ▁the ▁identity ▁of ▁total ▁sur plus ▁value ▁and ▁total ▁prof its ) ▁cannot ▁be ▁maintained ▁simultaneously ▁with ▁the ▁assumption ▁that ▁the ▁rate ▁of ▁profit ▁on ▁production ▁capital ▁is ▁the ▁same ▁for ▁all ▁indust ries — to ▁create ▁a ▁consistent ▁theory ▁requires ▁making ▁additional ▁assumptions . ▁ ▁Although ▁this ▁is ▁often ▁over look ed ▁by ▁econom ists ▁Marx ▁himself ▁used ▁a ▁uniform ▁rate ▁of ▁profit ▁for ▁all ▁indust ries ▁in ▁Capital ▁Vol . ▁ 3 ▁only ▁for ▁mod elling ▁purposes , ▁to ▁show ▁in ▁a ▁simple ▁way ▁how ▁the ▁r uling ▁profit ▁rates ▁on ▁capital ▁impact ed ▁on ▁the ▁development ▁of ▁production ▁system , ▁and ▁he ▁explicitly ▁denied ▁that ▁a ▁uniform ▁rate ▁of ▁profit ▁obtained ▁in ▁reality ; ▁he ▁only ▁argued ▁that ▁at ▁any ▁time ▁there ▁would ▁exist ▁an ▁average ▁" min imum ▁acceptable " ▁profit ▁rate ▁on