▁assume ▁that ▁light ▁has ▁a ▁constant ▁speed ▁and ▁that ▁this ▁speed ▁is ▁the ▁same ▁in ▁all ▁directions . ▁Without ▁this ▁post ulate ▁it ▁would ▁be ▁impossible ▁to ▁infer ▁the ▁speed ▁of ▁light ▁from ▁astronom ical ▁observations , ▁as ▁Ole ▁R ø mer ▁did ▁based ▁on ▁observations ▁of ▁the ▁mo ons ▁of ▁Jup iter . ▁Po inc ar é ▁also ▁noted ▁that ▁the ▁propag ation ▁speed ▁of ▁light ▁can ▁be ▁( and ▁in ▁practice ▁often ▁is ) ▁used ▁to ▁define ▁simultane ity ▁between ▁spatial ly ▁separate ▁events . ▁He ▁concluded ▁by ▁saying ▁that ▁" The ▁simultane ity ▁of ▁two ▁events , ▁or ▁the ▁order ▁of ▁their ▁succession , ▁the ▁equality ▁of ▁two ▁dur ations , ▁are ▁to ▁be ▁so ▁defined ▁that ▁the ▁en unci ation ▁of ▁the ▁natural ▁laws ▁may ▁be ▁as ▁simple ▁as ▁possible . ▁In ▁other ▁words , ▁all ▁these ▁rules , ▁all ▁these ▁definitions ▁are ▁only ▁the ▁fruit ▁of ▁an ▁un cons cious ▁opportun ism ." ▁ ▁In ▁some ▁other ▁papers , ▁Po inc ar é ▁( 1 8 9 5 , ▁ 1 9 0 0 b ) ▁argued ▁that ▁experiments ▁like ▁that ▁of ▁Michel son – Mor ley ▁show ▁the ▁impos s ibility ▁of ▁detect ing ▁the ▁absolute ▁motion ▁of ▁matter , ▁i . e ., ▁the ▁relative ▁motion ▁of ▁matter ▁in ▁relation ▁to ▁the ▁a ether . ▁He ▁called ▁this ▁the ▁" pr inci ple ▁of ▁relative ▁motion ". ▁In ▁the ▁same ▁year ▁he ▁interpreted ▁Lor ent z ' s ▁local ▁time ▁as ▁the ▁result ▁of ▁a ▁synchron