▁contradiction ▁if ▁the ▁past ▁becomes ▁different ▁from ▁the ▁way ▁it ▁is . ▁The ▁par ado x ▁is ▁commonly ▁described ▁with ▁a ▁person ▁who ▁travel s ▁to ▁the ▁past ▁and ▁k ills ▁their ▁own ▁grand father , ▁prevents ▁the ▁existence ▁of ▁their ▁father ▁or ▁mother , ▁and ▁therefore ▁their ▁own ▁existence . ▁Philosoph ers ▁question ▁whether ▁these ▁par ado xes ▁make ▁time ▁travel ▁impossible . ▁Some ▁philosoph ers ▁answer ▁the ▁par ado xes ▁by ▁arg uing ▁that ▁it ▁might ▁be ▁the ▁case ▁that ▁back ward ▁time ▁travel ▁could ▁be ▁possible ▁but ▁that ▁it ▁would ▁be ▁impossible ▁to ▁actually ▁change ▁the ▁past ▁in ▁any ▁way , ▁an ▁idea ▁similar ▁to ▁the ▁proposed ▁Nov ik ov ▁self - cons ist ency ▁principle ▁in ▁physics . ▁ ▁Ont ological ▁par ado x ▁ ▁Compos s ibility ▁ ▁According ▁to ▁the ▁philosoph ical ▁theory ▁of ▁comp oss ibility , ▁what ▁can ▁happen , ▁for ▁example ▁in ▁the ▁context ▁of ▁time ▁travel , ▁must ▁be ▁we ighed ▁against ▁the ▁context ▁of ▁everything ▁rel ating ▁to ▁the ▁situation . ▁If ▁the ▁past ▁is ▁a ▁certain ▁way , ▁it ' s ▁not ▁possible ▁for ▁it ▁to ▁be ▁any ▁other ▁way . ▁What ▁can ▁happen ▁when ▁a ▁time ▁travel er ▁vis its ▁the ▁past ▁is ▁limited ▁to ▁what ▁did ▁happen , ▁in ▁order ▁to ▁prevent ▁logical ▁contradict ions . ▁ ▁Self - cons ist ency ▁principle ▁ ▁The ▁Nov ik ov ▁self - cons ist ency ▁principle , ▁named ▁after ▁Ig or ▁D mit rie v ich ▁Nov ik ov , ▁states