Now here's a problem: if the future is already settled---that is, if what I call "realism about the future" is true---then the reasons we gave for saying that it's impossible for the time traveler to change the past seem to extend to us, too, and to show that it's impossible for us to change the future.
Consider the following cases:
As a matter of fact, then, the murder plot you overhear will succeed, or it won't. Whichever outcome is true is already true. So it doesn't seem like anything you do can change this outcome--just as nothing you did while time traveling to the past could change the past.
This is a piece of fatalist reasoning. The fatalist says that all the events that are in fact going to happen are fated to happen, in the sense that there's nothing we can do to stop them from happening.
More precisely, we can take the fatalist to be someone who accepts the following thesis, which I'll call the Impotence Thesis:
(Past-Tense Version) If E happened, there's nothing you could have done that would have prevented E. It wasn't in your power to make something else happen.
(Future-Tense Version) If E is going to happen, then again, it's not in your power to make E not happen. There's nothing you can do to change or influence what will happen.
That fatalist is someone who believes that the Impotence Thesis is true because the future is already settled. That is, the fatalist is someone who thinks that Realism about the Future entails the Impotence Thesis. Suppose it's already true today that the murder plot you overhear will succeed on Jan 31 2007. The fatalist argues as follows: Well, if it's already true that this murder will occur on Jan 31 2007, then nothing I do can prevent it from occurring. If I try to prevent the murder, then I must fail--since it's already true that come Jan. 31 2007, this murder will take place. So it's impossible for me to prevent it. (And if it's already false today that the murder plot will succeed, then nothing I do can get that murder to take place.)
Let's formalize the fatalist's reasoning as follows:
Parallel reasoning would show that there's no point taking any steps to bring E about.
What should we think about this argument? You might want to reject the fatalist's assumption that the future is already settled. We can call that assumption Realism about the Future. It's embodied in steps 1+2+4 in the argument. But I want us to grant Realism about the Future, and see what follows from that. Let's see if Realism about the Future really does entail what the fatalist says it entails.
We want to look closely at steps 3 and 5 in this argument.
The fatalist's reasoning is tempting when you're talking about outcomes which are, intuitively, largely outside your control.
But this argument is supposed to apply to every future outcome, even outcomes that we would have intuitively thought to be within your control. Consider the following application of the fatalist's reasoning:
Either this letter will get mailed or it won't. If it's already true that it won't get mailed, then any steps I take to mail it (such as taking it to the mailbox) would be ineffectual. If it's already true that it will get mailed, then any steps I take to mail it would be superfluous. So there's no point taking it to the mailbox.
Clearly something has gone wrong in this piece of reasoning. The question is: what has gone wrong?
To figure this out, let's go back and think about the movie Twelve Monkeys. The fatalist says that we're in the same boat with respect to the future as the time traveler is in with respect to the past. We're both powerless. Everything is inevitable, and nothing we do makes any difference.
Well, let's see if that's right. Let's ask ourselves, in the movie Twelve Monkeys, could James Cole change things in 1996 or couldn't he?
On the one hand, it looks like Cole can't change what happens. The movie emphasizes this again and again. But on the other hand, Cole quite clearly has an effect on the events in 1996, doesn't he? His being there makes things happen which wouldn't have happened if he weren't there:
Answer: these questions are ambiguous. We've been arguing that Cole can't revise the past. The past can't happen first one way, and then Cole goes back and changes it to happen a different way. That doesn't make sense. It is not possible to revise the past in this way. So if that's what we mean by "changing things in 1996," then the answer is no, Cole can't do that.
But there's something else you might mean by "changing things in 1996." You might mean: does Cole influence or affect what happens in 1996? Does he make a difference to how 1996 evolves? Yes, this he does do. The path the past took depended partly on what he did when he was there. If Cole hadn't been there and done the things he did, 1996 would have evolved differently than it actually did.
Here we're making a counterfactual claim. This is a claim about what would have happened, if certain things had been different. Earlier in the term we talked about dispositions. To say that a glass has a certain disposition, like fragility, is to make a counterfactual claim about the glass. It's to say that the glass would break, if it were struck. Perhaps the glass never in fact gets struck, and never in fact breaks. But it can still be fragile. It can still be such that if you had struck it, it would have broken.
Similarly, we know that Cole will in fact do certain things, and 1996 will in fact turn out a certain way. And the fact that the past has already turned out that way entails that it can't happen again, a different way. But it doesn't at all entail that the past had to happen that way. It can very well be that if Cole had acted differently, then the past would have turned out differently. And this in fact seems to be the case. Cole's actions do seem to make some difference to how 1996 turns out.
This cannot happen: first 1996 happens one way, and then Cole goes back and makes it happen a different way. Cole cannot revise 1996 in this way.
But Cole can and does: bring it about that 1996 happens differently than it would have happened if he hadn't been there.
Let's see if this helps us with the fatalist's argument about mailing the letter.
Hence, the fact (if it is a fact) that:
If the letter won't get mailed, all that follows is that:
So this move in the fatalist's argument is also faulty.
I think the fatalist is right to compare our situation to the time traveler's situation. But he's wrong to think that any fatalist result follows from this. There is only one past, and only one future. So neither the time traveler nor I can revise time. Whatever happens happens. And it only happens once; we don't get a chance to rewrite the story. But the time traveler and I both make a difference to what happens. He causes the past to happen differently than it would have happened if he hadn't acted. And I influence the future in the same way.
Of course, there will be events in the past that the time traveler fails to prevent; and there will be events in the future that we fail to prevent. Some of these will be events beyond our control. They still would have happened, no matter how we acted. For example, we can't influence the moon's orbit. But not all future events are like that. It's not the case for all future events that they still would have happened, no matter what we did. Some of these events will be such that, if we had acted differently, those events would have been prevented. (For instance, I could have prevented the letter from being mailed.)
To summarize. We had this thesis:
Impotence Thesis: If E is going to happen, then you lack the power or ability to prevent E from happening, or to make something else happen, instead.
The fatalist has been arguing for this thesis in a certain way. He thinks there's already a fact of the matter about how the future will turn out. (We called this view Realism about the Future.) And the fatalist thinks that that shows the Impotence Thesis to be true.
I've been arguing that Realism about the Future does not, by itself, show that the Impotence Thesis is true.
There may be other, better reasons to think we're Impotent. We haven't shown that the Impotence Thesis is false; we've haven't shown that we do have the power to do anything other than we in fact do. We've just shown that the fatalist's argument for the Impotence Thesis is unsuccessful.
Perhaps other sorts of considerations, e.g. considerations about how the past causally determines the future, will turn out to show that the Impotence Thesis is true, anyway. That's something we will have to consider more carefully.