time travel the grandfather paradox i

the grandfather paradox 1. If time travel is possible, then one can commit grampatricide before one’s father is conceived. 2. One cannot commit grampatricide before one’s father is conceived. 3. [So] Time travel is not possible.

the retro-suicide paradox 1. If time travel is possible, then one can commit retro-suicide. 2. One cannot commit retro-suicide. 3. [So] Time travel is not possible.

the altered past paradox 1. If time travel is possible, then one can change the past. 2. One cannot change the past. 3. [So] Time travel is not possible.

the altered past paradox The Law of Non-Contradiction: For any proposition P, it is not the case that (P & not-P).

the altered past paradox The Change Principle: Necessarily, for any X, X changes only if there is some property P such that one of X’s temporal parts has P and one of X’s temporal parts lacks P.

the altered past paradox 1. If time travel is possible, then one can change the past. 2. One cannot change the past. 3. [So] Time travel is not possible.

lewis’s puzzle A. Tim can kill Grandfather. B. Tim cannot kill Grandfather.

lewis’s puzzle The Compossibility Principle: A sentence of the form “A can X” is true in context C iff A’s Xing is compossible with the facts that are relevant in context C.

lewis’s puzzle A. Tim can kill Grandfather. B. Tim cannot kill Grandfather.

lewis’s puzzle A. Tim’s killing Grandfather is compossible with the R facts. B. Tim’s killing Grandfather is not compossible with the R+ facts.