, ▁and ▁on ▁the ▁other ▁side ▁the ▁elect romagnet ic ▁origin ▁of ▁all ▁forces ▁is ▁assumed . ▁Abraham ▁showed , ▁that ▁both ▁assumptions ▁were ▁in compatible , ▁because ▁in ▁Lor ent z ' s ▁theory ▁of ▁the ▁contract ed ▁electrons , ▁non - elect ric ▁forces ▁were ▁needed ▁in ▁order ▁to ▁guarantee ▁the ▁stability ▁of ▁matter . ▁However , ▁in ▁Abraham ' s ▁theory ▁of ▁the ▁rig id ▁electron , ▁no ▁such ▁forces ▁were ▁needed . ▁Thus ▁the ▁question ▁arose ▁whether ▁the ▁Elect romagnet ic ▁conception ▁of ▁the ▁world ▁( compatible ▁with ▁Abraham ' s ▁theory ) ▁or ▁the ▁Rel ativity ▁Princi ple ▁( compatible ▁with ▁Lor ent z ' s ▁Theory ) ▁was ▁correct . ▁ ▁In ▁a ▁September ▁ 1 9 0 4 ▁lecture ▁in ▁St . ▁Louis ▁named ▁The ▁Princi ples ▁of ▁Mathemat ical ▁Physics , ▁Po inc ar é ▁drew ▁some ▁consequences ▁from ▁Lor ent z ' s ▁theory ▁and ▁defined ▁( in ▁modification ▁of ▁Gal ile o ' s ▁Rel ativity ▁Princi ple ▁and ▁Lor ent z ' s ▁Theorem ▁of ▁Cor respond ing ▁States ) ▁the ▁following ▁principle : ▁" The ▁Princi ple ▁of ▁Rel ativity , ▁according ▁to ▁which ▁the ▁laws ▁of ▁physical ▁phen omena ▁must ▁be ▁the ▁same ▁for ▁a ▁station ary ▁observer ▁as ▁for ▁one ▁carried ▁along ▁in ▁a ▁uniform ▁motion ▁of ▁translation , ▁so ▁that ▁we ▁have ▁no ▁means , ▁and ▁can ▁have ▁none , ▁of ▁determ ining ▁whether ▁or ▁not ▁we ▁are ▁being ▁carried ▁along ▁in ▁such ▁a ▁motion ." ▁He ▁also