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Chapter 1

PHIL-020 Chapter Notes - Chapter 1: Temporal Parts, Time Travel

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Mattingly James

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David Lewis
The Paradoxes of Time Travel
Overall argument:
Time travel is possible: “what goes on in a time travel story may be a possible pattern of events in four-
dimensional space-time with no extra time dimension” (4). This of course, is assuming that there is no
other branch of time that runs parallel with our own. (At the end of the essay, he states that if time was
indeed two-dimensional, there would exist multiple branches in which you could succeed doing something
in the past, but still not change it). Additionally, a time traveller cannot change the past because the real
past is in alignment with his/her own personal past, and since the traveler is already a product of his
personal time, it is impossible for him to physically change the past. Lewis uses the example of a boy
named Tim going into the past to kill his Grandfather, but who does not succeed because the
assassination is logically impossible, considering his personal time.
Important words: compossible, external time, personal time, change
Compossible (adj.): (of one thing) compatible or possible in conjunction with another.
External time: time in general
Personal time: “that which occupies a certain role in the pattern of events that comprise” one’s life
Change: “qualitative difference between temporal parts of something”
EX: the you of the past and the current you
~ Reading Quotes/Notes (Paraphrasing) ~
-Lewis muses that time travel is possible, and that he will hopefully convince the reader of this
through his essay (1)
-“The paradoxes of time travel are oddities, not impossibilities” (1)
-time travel = relies on discrepancies between time and itself
-one’s departure into the past and his arrival are separated by two unequal amounts of time (1) Why?
-set aside the possible answer that time is two dimensional & the possibility that an event might
be momentary along one time dimension but divisible along the other (2)
-while the life of a common person occupies a straight line across the plane of time, the life of a
time traveller occupies a bent path of varying slope (1)
-to answer this, must distinguish time (external time) from the personal time of a particular time
traveler (2)
-time is one dimension of our four-dimensional world
-different temporal parts of something: the result of change between stages
-for example, two temporal parts of you: the one that started reading and the one that ended
reading this (2)
-change = “qualitative difference between temporal parts of something”
-therefore, something that does not have temporal parts cannot change; for example, numbers
-Note: “Cambridge change” refers to pretty much every change: it says that a number can
change, such as the inflation of a price, etc. But these changes are “not genuine changes” (2)
-definition of personal time: that which occupies a certain role in the pattern of events that comprise
the time travelers life
-liken intervals of external time to distance as the crow flies, while intervals of personal time are
distances along a winding path. Important note: the personal time is not a second dimension of time
-what unites the stages of a time traveler while traveling is the same sort of mental continuity and
connectedness that unites anyone else; a common person is connected with respect to external time,
while the time traveler is connected only with respect to his own personal time (3)
-case of counterfeit time travel: Fred being created out of thin air, died, and then someone named
Sam was born that looked and perfectly resembled Fred
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