  =   C   ( Y   ), ▁i . e . ▁to ▁treat ▁the ▁prop ens ity ▁to ▁consume ▁as ▁independent ▁of ▁the ▁price ▁level . ▁Ke yn es ▁shows ▁that ▁he ▁is ▁conscious ▁that ▁this ▁is ▁an ▁approximation ▁in ▁Point ▁ 1 ▁of ▁§ II ▁of ▁Chapter ▁ 8 . ▁ ▁The ▁prop ens ity ▁to ▁save ▁is ▁the ▁complement ▁of ▁the ▁prop ens ity ▁to ▁consume : ▁S ( Y ) ▁= ▁Y ▁- ▁C ( Y ). ▁It ▁plays ▁an ▁important ▁role ▁in ▁Ke yn es ' s ▁formal ism ▁as ▁one ▁of ▁the ▁ex ogen ous ▁quantities ▁determ ining ▁the ▁state ▁of ▁the ▁economy . ▁The ▁interaction ▁of ▁these ▁quantities ▁is ▁expressed ▁as ▁a ▁pair ▁of ▁simultane ous ▁equations ▁in ▁w age ▁units , ▁in ▁which ▁Y ▁and ▁r ▁( total ▁income ▁in ▁w age ▁units ▁and ▁the ▁interest ▁rate ) ▁are ▁considered ▁to ▁be ▁the ▁sole ▁variables . ▁The ▁man oe uv re ▁by ▁which ▁Ke yn es ▁approxim ates ▁C ▁as ▁C ( Y ) ▁rather ▁than ▁as ▁a ▁function ▁of ▁Y ▁and ▁p ▁together ▁is ▁essential ▁to ▁avoid ing ▁the ▁occurrence ▁of ▁p ▁in ▁these ▁equations . ▁ ▁Li qu id ity ▁preference ▁ ▁Ke yn es ' s ▁initial ▁( Ch apter ▁ 1 3 ) ▁model ▁of ▁liquid ity ▁preference ▁cons iders ▁the ▁demand ▁for ▁money ▁to ▁depend ▁sole ly ▁on ▁the ▁interest ▁rate . ▁This ▁is ▁purely ▁mon et ary : ▁the ▁liquid ity ▁preference ▁can ▁be ▁written ▁L   (