1. time travel vranas on changing the past

2. 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.

3. peter vranas

4. definitions Someone affects the past iff he does something that has a causal effect on the past. Someone changes the past iff he does something that makes the past qualitatively different from what it is.

5. definitions It is possible to transform the past iff there is a possible world W and time T such that: Someone does something at T which makes the past in W different from the past in W. It is possible to replace the past iff there is a possible world W and time T such that: Someone does something at T which makes the past in W different from the past in the actual world.

6. vranas’s argument 1. If time travel is possible, then it is possible to affect the past. 2. If it is possible to affect the past, then it is possible to replace the past. 3. If it is possible to replace the past, then it is possible to change the past. 4. [So] If time travel is possible, then it is possible to change the past.

7. definitions Qualitative change occurs when something has different properties (or “qualities”) at different times. Substantial change occurs when different things (or “substances”) exist at different times. Intrinsic change occurs when something has different intrinsic properties at different times. Extrinsic change occurs when something has different extrinsic properties at different times.

8. definitions It is possible to change the past in the qualitative sense iff there is a possible world W and times T1 and T2 such that: Someone does something in W that makes the past in W have different properties at different times. It is possible for someone to change the past in the substantial sense iff there is a possible world W and times T1 and T2 such that: Someone does something in W that makes different pasts exist at different times. It is possible to “change the past” in the vranas sense iff there is a possible world W and time T such that: Someone does something at T which makes the past in W different from the past in the actual world.

9. definitions It is possible to “change the unchangable monument” in the vranas sense iff there is a possible world W and time T such that: Someone does something at T which makes the unchangeable monument in W different from the unchangeable monument in the actual world. It is possible to “change the past” in the vranas sense iff there is a possible world W and time T such that: Someone does something at T which makes the past in W different from the past in the actual world

10. vranas’s argument 1. If time travel is possible, then it is possible to affect the past. 2. If it is possible to affect the past, then it is possible to replace the past. 3. If it is possible to replace the past, then it is possible to change the past. 4. [So] If time travel is possible, then it is possible to change the past.