Post on 30-May-2018
8/14/2019 Phil 265 Lewis Paradoxes Micro Essay
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Nathaniel Kan - Philosophy 265 "Paradoxes of Time Travel" Microessay
In "The Paradoxes of Time Travel," David Lewis proposes that time travel is not logically
inconsistent. Lewis rejects the two dimensional theory of time, where time1 flows at a rate of time2, and
instead accepts the manifold theory of time. He defines two different measures of time: external time, the
time which orders moments in the world, and personal time, which is the time that the time traveler
undergoes. Time travel, then, occurs when there is a difference between external time and personal time.
Although a time traveler might seem to be two separate people, what unites the two is the mental
connectedness and continuity that Lewis assigns to an individual in "Survival and Identity." How is time
travel any different then the spontaneous death of a person at some moment and the creation of an identical
person at a different moment? Lewis believes that because the two people are not causally linked (at least not
in the right way) this is not time travel, but coincidence.
One anomaly is the possible existence of backwards causation and/or causal loops. A causal loop
might occur if a time traveler were to travel back into the past in external time and teach his previous self
how to build the time machine. How can such a loop exist? Lewis argues that we accept certain facts as
inexplicable, such as the Big Bang or the infinite past of the universe, and then why should we not accept an
inexplicable causal loop?
The paradox most contradictory to the idea of time travel is the time traveler, Tim, who goes back in
time to kill his grandfather in 1921. Lewis believes that Tim cannot kill Grandfather, because it creates a
logical impossibility. There is only one 1921, and if Grandfather wasnt killed in it and then is killed in it,
there is a contradiction. However, we can say that Tim can kill Grandfather because to say that something
can happen means that it is compossible with certain facts, in this case Tim's location, intent, etc. If we
suppose Tim succeeds and hold the rest of the story fixed, we get a contradiction, however if you make any
counterfactual supposition and hold everything else fixed you get a contradiction.
If we were to consider branching time, then we might think that Tim could succeed. In this case,
however, Tim would be not only traveling in time, but also in branches, and thus there would be two parallel
stories: one where Grandfather lived and one where he did not.