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Time Travel
Visiting the Past

In the Fall of 1993, in the Critical Thinking course (PHIL 001), we were using the textbook Logical Reasoning  (Wadsworth), by Bradley Dowden. We had been working through the book, chapter by chapter. On Oct. 31, I read p. 202, which contains the following argument:

 Nobody has ever built a time machine that could take a person back to an earlier time. Nobody should be seriously trying to build one, either, because a good argument exists for why the machine can never be built. The argument goes like this. Suppose you did have a time machine right now, and you could step into it and travel back to some earlier time. Your actions in that time might then prevent your grandparents from ever having met one another. This would make you not born, and thus not step into the time machine. So, the claim that there could be a time machine is self-contradictory.

That evening, I sent an email letter to Bradley Dowden (with whom I had been corresponding occasionally during the previous few weeks). Below, with his permission, is a transcript of our exchange.

Swartz to Dowden

October 31, 1993

I have tonight read your chapter nine.

I disagree with your argument at the top of p. 202, the one which alleges to show that (time) travel into the past is logically impossible. I would like to suggest that your argument commits a modal fallacy.

Since it is possible that someone should have prevented your grandparents from having met one another, and since it is impossible for you to travel into the past and to have prevented your grandparents from having met one another, you conclude that it is thus impossible to travel into the past. Let "P" stand for "preventing your grandparents from meeting" and "T" stand for "travel into the past" (patched up as needed to be proper statements). Then your argument is

P
~(T & P)

therefore ~T
The argument is invalid. From the conjoint impossibility of P and T, and the possibility of P, the impossibility of T does not  follow. (Just to drive the point home, now let "T" stand for "the coffee table is four-sided" and let "P" stand for "the coffee table is six-sided".)

What is  logically impossible is that BOTH one travels into the past  AND changes the past  (from what it was). But so long as one does not change the past, there is no logical contradiction in positing travel into the past.

Below I reproduce section 8.11 (pp. 224-227) from my book Beyond Experience: Metaphysical Theories and Philosophical Constraints  (Toronto: Toronto University Press), 1991. You may want, also, to look at Possible Worlds: An Introduction to Logic and Its Philosophy, by Raymond Bradley and myself (Indianapolis: Hackett), 1979, p. 25, exercise 4, and p. 333, part C. David Lewis has written a fine paper on time travel. I have the exact reference on campus. I have another book here at home that lists a paper by Lewis. If, when I get to campus tomorrow, I find that there is more than one Lewis paper on time travel and that the one I list here is not the one I wanted, I will send along the corrected reference. In the meantime, I think, the paper is "The Paradoxes of Time Travel", in American Philosophical Quarterly, vol. 13, 1976, pp. 145-152.

Regards,

–– Norman

Dowden to Swartz

Nov. 1, 1993

Norman,

You make some interesting points about time travel, but let me offer a counter.

1. The formal modal argument is invalid, but my argument doesn't properly translate into that modal argument.

2. I do not agree that my reasoning on p. 202 is as follows: "Since it is possible that someone should have prevented your grandparents from having met one another, and since it is impossible for you to travel into the past and to have prevented your grandparents from having met one another, you conclude that it is thus impossible to travel into the past." That isn't why I conclude that time travel into the past is impossible. Why I do conclude this will be clearer in a moment.

3. I do agree that I am using the same old argument that you discuss in your book in the following passage:"If you could travel backward in time, then you could encounter yourself when you were a youngster. Even if you are not normally homicidally inclined, it is at least theoretically possible that you kill that youngster. But if you did, then you would not have grown up to have reached the age when you traveled back in time. Thus there would be a contradiction: you both would and would not have traveled backward in time. Since the story involves a contradiction, it is logically impossible to travel backward in time."

4. I agree that your analysis of time travel into the past does show that "If one travels into the past, then one does not change the past."

5. However, what we MEAN, or at least what I and most other people I talk with mean, by travel into the past, is travel in which the past does change. Statements that are true are made false. To travel into the past as a disembodied watcher of past events is not to travel at all.

6. You believe time travel is possible. OK. Let's suppose you have your time machine and you go back. For example, suppose you step into a time machine tomorrow and go say hello to your great grandparents. This supposition leads immediately to a contradiction because it is true that you never did say hello to your great grandparents. To say hello is to change the past, but as you will certainly agree, to make the true be false is absurd, and to change the past is logically impossible. Whatever is true is true, period. Therefore, the only way to travel into the past is to do so in a way that changes nothing. But to travel this way is thereby not to travel into the past at all because travel into the past requires changing the past.

7. So I stand by my argument on p. 202.

Swartz to Dowden

November 2, 1993

BD> 5. However, what we MEAN, or at least what I and most other
BD> people I talk with mean, by travel into the past, is travel
BD> in which the past does change. Statements that are true are
BD> made false. To travel into the past as a disembodied
BD> watcher of past events is not to travel at all.

This is too easy, and – no offense intended – a bit of a cheat. Compare with this:
It is logically impossible to build a car that will take one from Sacramento to Cleveland. What we MEAN, or at least what I and most other people I talk with mean, by travel to distant places, is travel in which the distant place is changed. Statements that are true are made false. To travel to a distant place as a disembodied watcher of distant events is not to travel at all.
Of course a time machine which allowed one to change  the past is  logically impossible, but making that the (unrealizable) goal is to trivialize the problem, to pose it in a question-begging manner, to stack the deck. Even if one does stack the deck in this way, there still remains the question: Is it logically possible to build a time machine which allows travel into the past where one does not  change the past? And the answer to this latter, nontrivial, version of the question is: yes. Robert Heinlein, for example, has described the manner brilliantly in his long book Time Enough for Love  and in his short story "All You Zombies". I defy anyone to find a logical contradiction in those tales.

When I  argue for the logical possibility of travel into the past, I do not mean as a disembodied watcher of past events. I mean as a 'real' participant in the activity of the past. Suppose a visitor were to arrive here and now from the year 2045. He shakes my hand, and then sits and chats with me about what is in store during the next 52 years. I take notes and record them in my diary. A year (1994) from now, I even publish some of these notes. The visitor from the future (year 2045) has not changed the past (i.e. the past relative to the year 2045): he has contributed to making the past just the way it was. By traveling back to the year 1993, he caused certain events to occur in 1993 and in 1994. Nothing was changed from the way it was; but the past was changed from the way it would have been if he had not traveled back from 2045 to 1993. Nothing true is made false; there is no logical contradiction.

BD> 6. You believe time travel is possible. OK. Let's suppose
BD> you have your time machine and you go back. For example,
BD> suppose you step into a time machine tomorrow and go say
BD> hello to your great grandparents. This supposition leads
BD> immediately to a contradiction because it is true that you
BD> never did say hello to your great grandparents. To say
BD> hello is to change the past, but as you will certainly
BD> agree, to make the true be false is absurd, and to change
BD> the past is logically impossible. Whatever is true is true,
BD> period. Therefore, the only way to travel into the past is
BD> to do so in a way that changes nothing. But to travel this
BD> way is thereby not to travel into the past at all because
BD> travel into the past requires changing the past.

To be sure, there are ways of telling time travel stories (your way immediately above) that are  self-contradictory. But to prove impossibility, it is not sufficient to tell one  story in which there is a self-contradiction, one must show something far stronger, viz. that every  story in which there is time travel harbors a contradiction. You have not done this. Heinlein and many others have offered stories in which there are no contradictions. And all it takes to demonstrate possibility is one story free of contradiction. (Many philosophers misunderstand the methodology of possible-worlds tales. Quinton, for example, got it wrong in his "Spaces and Times", Philosophy, vol. 37 no. 140 (Apr. 1962), pp. 130-47. I explain his error, and the correct methodology, on pp. 218-219 in my Beyond Experience  (referred to earlier).)

Remember the logic of possibility ( ) is just the logic of existence [ (x) ] extended to the set of all possible worlds. Failure to find time travel in one  possible world does not show its impossibility, any more than failing to find my wristwatch in the bedroom would show that it does not exist. To be sure there are (some) scenarios describing time travel which are self-contradictory. (You just gave one such, in saying that you both said hello to your grandparents and did not.) But one proves the logical impossibility of time travel only if one shows that every  story of time travel is logically self-contradictory. There is no self-contradiction in the little story I just told about the visitor from 2045; there are no self-contradictions in the stories I just mentioned by Robert Heinlein. In these stories, the time travelers 'really' walk about, observe, and are actors on the scene, in the past; however, they change nothing. They no more bring about contradictions than you and I do by doing whatever it is we do today.

BD> Therefore, the only way to travel into the past is
BD> to do so in a way that changes nothing.

Yes. I agree completely. But I would also add "Ditto for travel in space". Travel in space is similarly constrained by the law of non-contradiction.

BD> But to travel this way is thereby not to travel into the past
BD> at all because travel into the past requires changing the
BD> past.

This is too strong, and it is question-begging. The corresponding argument about space would 'show' (illicitly) that travel in space is impossible.

–– Norman

Dowden to Swartz

Nov. 2, 1993

Norman,

I found your argument about the visitor from 2045 very convincing. I guess I've never thought very seriously about the issue of time travel. Yes, I now believe it is logically possible to build a time machine which allows travel into the past where one does not change the past. This would be 'real' time travel, not just a disembodied existence passively viewing the scene. The person could contribute to making the past just the way it was.

Previously when I've thought about time travel into the past I've thought of that kind of travel that meets the demand "Oh, I wish I could go back and make the Toronto Blue Jays lose the World Series, or go back and make Adolf Hitler slip on a banana peel and die at the age of seven." That sort of time travel, in which the past does get changed, is logically impossible, and that is the only kind of time travel I was considering when I wrote page 202 of my textbook. So I'll change my ways in the future. I'll change the first sentence at the top of page 202 to say "Nobody has ever built a time machine that could take a person back to an earlier time to change what has happened." No, I believe the whole paragraph should be rewritten.

Thanks for the discussion about this topic of time travel.