▁be ▁prof itable ▁as ▁a ▁function ▁of ▁the ▁interest ▁rate . ▁It ▁is ▁introduced ▁in ▁Chapter ▁ 1 1 ▁of ▁the ▁General ▁Theory   ▁with ▁no ▁mention ▁of ▁units . ▁The ▁definition ▁is ▁of ▁a ▁quantity ▁in ▁real ▁terms : ▁it ▁spec ifies ▁“ by ▁how ▁much ▁invest ment ... ▁will ▁have ▁to ▁increase ▁within ▁the ▁period , ▁in ▁order ▁that ▁its ▁marg inal ▁efficiency ▁should ▁fall ▁to ▁any ▁given ▁level ” ▁( p 1 3 6 ): ▁in ▁other ▁words , ▁how ▁much ▁new ▁equipment ▁will ▁give ▁a ▁return ▁above ▁that ▁level . ▁This ▁is ▁viewed ▁as ▁an ▁ex ogen ous ▁quantity . ▁ ▁The ▁role ▁of ▁the ▁schedule ▁of ▁the ▁marg inal ▁efficiency ▁of ▁capital ▁in ▁Ke yn es ' s ▁system ▁is ▁through ▁the ▁IS ▁equation , ▁which ▁as ser ts ▁that ▁, ▁i . e . ▁that ▁the ▁amount ▁of ▁invest ment ▁I ( r ) ▁which ▁will ▁take ▁place ▁at ▁interest ▁rate ▁r ▁is ▁equal ▁to ▁the ▁amount ▁of ▁saving ▁which ▁will ▁take ▁place ▁at ▁income ▁Y . ▁This ▁equation ▁is ▁stated ▁in ▁w age ▁units ▁and ▁combined ▁with ▁the ▁L M ▁equation ▁( like wise ▁in ▁w age ▁units ); ▁and ▁the ▁two ▁equations ▁together ▁are ▁de emed ▁to ▁determine ▁the ▁values ▁of ▁the ▁two ▁variables ▁Y ▁and ▁r . ▁Ke yn es ▁does ▁not ▁provide ▁an ▁explanation ▁of ▁how ▁the ▁schedule , ▁having ▁been ▁defined ▁in ▁real ▁terms , ▁can ▁be ▁treated ▁as ▁ex ogen ous ▁in ▁w age ▁units . ▁ ▁Inter pret ation ▁of