▁based ▁on ▁the ▁custom s ▁and ▁consent ▁of ▁King ▁John ' s ▁subjects ▁and , ▁since ▁there ▁was ▁no ▁Parliament ▁in ▁those ▁times , ▁neither ▁the ▁king ▁nor ▁the ▁bar ons ▁could ▁make ▁a ▁law ▁without ▁the ▁consent ▁of ▁the ▁people . ▁According ▁to ▁some ▁sources , ▁in ▁the ▁time ▁of ▁Edward ▁III , ▁" by ▁the ▁law ▁of ▁the ▁land " ▁had ▁been ▁subst it uted ▁" by ▁due ▁process ▁of ▁law ", ▁which ▁in ▁those ▁times ▁was ▁a ▁trial ▁by ▁twelve ▁pe ers . ▁ ▁During ▁the ▁mid - 1 4 th ▁century , ▁it ▁was ▁forb idden ▁that ▁persons ▁who ▁had ▁sat ▁on ▁the ▁present ing ▁j ury ▁( i . e ., ▁in ▁modern ▁parl ance , ▁the ▁grand ▁j ury ) ▁should ▁also ▁sit ▁on ▁the ▁trial ▁j ury ▁for ▁that ▁crime . ▁Med ieval ▁jur ies ▁were ▁self - in form ing , ▁in ▁that ▁individuals ▁were ▁chosen ▁as ▁jur ors ▁because ▁they ▁either ▁knew ▁the ▁parties ▁and ▁the ▁facts , ▁or ▁they ▁had ▁the ▁duty ▁to ▁discover ▁them . ▁This ▁sp ared ▁the ▁government ▁the ▁cost ▁of ▁fact - find ing . ▁Over ▁time , ▁English ▁jur ies ▁became ▁less ▁self - in form ing ▁and ▁re lied ▁more ▁on ▁the ▁trial ▁itself ▁for ▁information ▁on ▁the ▁case . ▁Jur ors ▁remained ▁free ▁to ▁investigate ▁cases ▁on ▁their ▁own ▁until ▁the ▁ 1 7 th ▁century . ▁Mag na ▁Cart a ▁being ▁forgotten ▁after ▁a ▁succession ▁of ▁bene vol ent ▁reign s ▁( or , ▁more