Lewis Grandfather Paradox

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The grandfather paradox is the idea that a time traveller that visits a time prior to their conception could change the past in such a way that prevents the conception from occurring (Study Guide p.9). This is seen as impossible as it leads to a circumstance where the time traveller no longer exist, therefore the act that causes her to not exist could not have been done. The grandfather paradox assumes that if time travel is possible, you can change the past. The law of contradiction tells us that contradictory statements cannot both be true at the same time. It is a contradiction to suggest that one can simultaneously exist and not exist (by virtue of killing one’s ancestor).

Lewis argues that time travel is logically possible by arguing

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