▁science ▁of ▁' ca us ality ' ▁as ▁used ▁in ▁natural ▁science ▁( H ans ▁K elsen , ▁Society ▁and ▁Nature , ▁ 1 9 4 6 , ▁pages ▁ 2 4 9 - 2 6 2 , ▁K eg an ▁Paul ▁Press ). ▁This ▁type ▁of ▁legal ▁science ▁would ▁be ▁' p ure ' ▁in ▁two ▁sens es ▁as ▁described ▁below . ▁ ▁Def ining ▁' p urity ' ▁of ▁legal ▁science ▁First ly , ▁it ▁would ▁be ▁' p ure ' ▁in ▁a ▁sense ▁analog ous ▁to ▁that ▁in ▁Kant ' s ▁Met aph ys ical ▁Found ations ▁of ▁Natural ▁Science . ▁It ▁would ▁set ▁out ▁a ▁pri ori ▁a ▁' p ure ▁part ' ▁of ▁legal ▁science , ▁consisting ▁of ▁a ▁framework ▁of ▁fundamental ▁concepts . ▁Then , ▁in ▁an ▁empir ical ▁part ▁of ▁legal ▁science , ▁this ▁framework ▁would ▁be ▁applied ▁to ▁empir ical ▁( soci ological , ▁historical ▁and ▁so ▁on ) ▁material ▁so ▁as ▁to ▁understand ▁that ▁material ▁as ▁' law '. ▁ ▁The ▁science , ▁in ▁both ▁of ▁its ▁parts , ▁would ▁also ▁be ▁' p ure ' ▁in ▁a ▁second ▁sense , ▁in ▁being ▁sole ly ▁descript ive — ex cluding ▁from ▁the ▁science ▁any ▁element ▁of ▁evaluation . ▁The ▁core ▁subject ▁matter ▁of ▁legal ▁science , ▁for ▁K elsen , ▁is ▁legal ▁norm s . ▁He ▁defines ▁a ▁' norm ' ▁as ▁' the ▁meaning ▁of ▁an ▁act ▁of ▁will '. ▁Thus , ▁for ▁K elsen ▁a ▁norm ▁does ▁not ▁necessarily ▁have ▁any ▁element