Editing Temporal paradox (section)

=== Logical impossibility ===
Even without knowing whether time travel to the past is physically possible, it is possible to show using [[modal logic]] that changing the past results in a logical contradiction. If it is necessarily true that the past happened in a certain way, then it is false and impossible for the past to have occurred in any other way. A time traveler would not be able to change the past from the way it ''is,'' but would only act in a way that is already consistent with what ''necessarily'' happened.<ref name="Norman">{{citation |author=Norman Swartz |title=Beyond Experience: Metaphysical Theories and Philosophical Constraints |date=2001 |url=https://www.sfu.ca/~swartz/beyond_experience/chap08.htm |pages=226–227 |publisher=University of Toronto Press}}</ref><ref>{{cite book |last1=Dummett |first1=Michael |title=The Seas of Language |date=1996 |publisher=Oxford University Press |isbn=0198236212 |edition=New |location=Oxford |pages=368–369}}</ref>

Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible. For example, the philosopher [[Bradley Dowden]] made this sort of argument in the textbook ''Logical Reasoning'', arguing that the possibility of creating a contradiction rules out time travel to the past entirely. However, some philosophers and scientists believe that time travel into the past need not be logically impossible provided that there is no possibility of changing the past,<ref name="NicholasSmith2"/> as suggested, for example, by the [[Novikov self-consistency principle]]. Dowden revised his view after being convinced of this in an exchange with the philosopher [[Norman Swartz]].<ref name="Swartz">{{cite web |author=Norman Swartz |year=1993 |title=Time Travel - Visiting the Past |url=https://www.sfu.ca/~swartz/time_travel1.htm |access-date=2016-04-21 |publisher=SFU.ca}}</ref>