1. Time Travel
Metaphysics 2019

2. David K. Lewis American philosopher (1941- 2001).
Student of W.V.O. Quine.
Important work in the foundations of logic, modal logic, probability theory, game theory, philosophy of language and mind, and metaphysics

3. Basic Discrepancies I get in my time machine in 2017.
I travel for one year.
I arrive in 1017.
= 1017?

4. Personal Time
Wristwatch time ≠ personal time. Aggregate – and not really time.

5. The Grandfather Paradox

6. Agustin Rayo Philosopher at MIT.
Works on philosophy of logic and language.

7. Augustin Rayo

8. Grandfather Paradox: Three Possibilities
Time travel is impossible
Time travel is possible, and normal rules apply  Grandfather paradox
Time travel is possible, but special rules apply  banana peel paradox

9. Possibility #1: Time Travel is Impossible
Can philosophy decide such things? (cf. Infinite chain of causes.)
Physics says it is possible.

11. Possibility #2: Grandfather Paradox Accepted
Possible solution: branching timelines

12. Possibility #3: Banana Peels
?v=YZRq3XxCZXo

13. Argument
Someone who has prepared carefully can kill their grandfather.
Bruno cannot kill his grandfather, though he has prepared carefully.
 Contradiction?

14. Lewis on Bananas
What I can do depends on an assumption of background facts.

15. Lewis on Bananas
A gibbon cannot speak Finnish. BACKGROUND FACTS: The vocal tract/ toungue/ lips/ etc. of a gibbon are incapable of making the sounds of the Finnish language.

16. Lewis on Bananas
David Lewis can speak Finnish. BACKGROUND FACTS: The vocal tract/ toungue/ lips/ etc. of David Lewis are capable of making the sounds of the Finnish language.

17. Lewis on Bananas
David Lewis can’t speak Finnish. BACKGROUND FACTS: Lewis knows no Finnish words or grammar.

18. Aside: Fatalism

19. Theological Fatalism
Suppose God knows that you will eat a hotdog for breakfast tomorrow. Then since what is known is true, you will eat a hotdog for breakfast tomorrow. Therefore, you *must* eat a hotdog for breakfast tomorrow.

20. Lewis on Bananas
What I can do depends on an assumption of background facts. If we assume BF: I have a range of options at the canteen; I’m capable of saying “I want a hotdog” and also “I don’t want a hotdog, I want….” – then I can fail to have a hotdog for breakfast.

21. Lewis on Bananas
What I can do depends on an assumption of background facts. If we assume BF: I have a range of options at the canteen… but we also assume that I do in fact order a hotdog… then I can’t logically do otherwise.

22. Lewis on Bananas
For Lewis, it is equally true that we can do what God knows we will not do, and that we can’t do it. It just depends on the background facts we are assuming.

23. Back to: Time Travel

24. Lewis on Bananas
Worry: it seems Bruno both can and can’t kill his grandfather.