Sunday, November 1, 2009

Why can't the past change?

If you're a B-theorist, it is no puzzle that the past can't change. It can't change because we are always in the same world, and so neither the past, nor the present nor the future can change. Today, let us suppose (I think correctly) that it is the case that on Wednesday it was raining. Could it tomorrow be the case that it wasn't raining on Wednesday? Not at all—for the very same world, the very same events, that make propositions true today is the one that we evaluate against tomorrow. The fact that the past can't change, thus, is a matter of mere logic—it just follows from the truth conditions for sentences.

But what if you're an A-theorist? So, you think that things will be objectively different tomorrow. Indeed, you already do think that some things about Wednesday will objectively change. For instance, while today (Saturday) Wednesday is objectively three days in the past, tomorrow it will objectively recede one more day into the past. So in fact we already have a change, but a change that the A-theorist doesn't mind. (Though she should.)

In any case, logic alone doesn't do the job. One way to see this is that some A-theorists actually think the future changes. Thus, today, it is false that either I am at Mass on November 8 or that I am absent from Mass on November 8. But come November 8, this disjunction will be true. But the clever tricks that open futurists use to make sense of an open future could be used, equally well, to make sense of an open past. (The parallel holds for B-theory. The B-theorist is committed to the claim that the future cannot change. This sounds fatalistic, but we must distinguish the ability to change the future from the ability to affect the future.)

In the setting of my earlier post on A-theory, the claim that the past cannot change corresponds fairly closely (and in fact exactly, if we assume a closed past) to the claim that the earlier-than relation is transitive. If today, a world where it rains on Wednesday is is past, tomorrow that world will also be past. So in the setting of that post, the explanatory challenge to the A-theorist is why the earlier-than relation E is transitive. The A-theorist who takes E to be fundamental can only say that it is a brute fact that it is necessarily transitive.

There may be A-theorists who can meet the challenge, however. Suppose that you think that there is a TimeShift operator which shifts tensed propositions time-wise. Thus, if p is the proposition that it is sunny, TimeShift(+1 day, p) is the proposition that in a day it'll be sunny. Suppose, further, we take worlds to be maximal consistent collections of propositions, or maximally specific consistent propositions. Then the TimeShift operator can also operate on worlds, and we can define E(w1,w2) to hold iff there is a t<0 such that w1=TimeShift(t,w2). Then it really is a matter of simple logic that E is transitive, and we have a perfectly good explanation of why the past cannot change.

Note, however, that an open-futurist cannot take this explanation. For her, the fixeity of the past remains a surd.

If this is right, then Tom Crisp is mistaken in taking the earlier-than relation between abstract times (which are just worlds in my terminology) to be primitive. An A-theorist should not say it's primitive—it needs explaining. Or at least I remember him taking it to be primitive, but my memory isn't so good.


Mike Almeida said...

It can't change because we are always in the same world, and so neither the past, nor the present nor the future can change.

How so? B-theory does not entail that the future is linear. It is consistent with B-theory that the future branches, and so might have been different from what it is. But if it might have been different, then it is possible to change the future. So, the future is F1. Can we make the future F2? Yes, we can, though we won't. Doesn't that make the future changeable?

Alexander R Pruss said...

An apple changes from green to red iff at t1 it is green and at a later t2 it is red. In general, change requires that something first have been one way, and now it be a different way. But if we made the future different, it would always have been in that "different way", and hence we still wouldn't have changed it.

So, of course, on the B-theory, it is possible for the future to be different from how it is. But this is not change, because change requires a succession. To change the future would require that at t1 the future have been one way, and at t2 that it be a different way. Only the open futurist can say that we change the future--and nobody should be an open futurist. Everybody else should deny that we can "change" the past, present or future, and instead say that we can affect the past, present or future.

I do suspect that when we loosely talk of the ability to change the future, we may sometimes mean only that we can affect it. But this is an improper use of the word "change", just as it is improper to say that if a timeless being creates spacetime, the timeless being changed nothingness into spacetime.

Mike Almeida said...

So, of course, on the B-theory, it is possible for the future to be different from how it is. But this is not change, because change requires a succession.

I thought you might go in this direction, which is why I spoke about the future, in general, changing. I didn't say the actual future changes or can. Each world has a unique actual future that cannot change. But why couldn't the future, broadly speaking, change? We can say that what changes is the unspecified future, just as what changes is the unspecified apple. The future takes on this particular content, and it might have had another content.

Mike Almeida said...

To put it another way, when you move from W to W*, the actual future does not change, though the actual future in W is different from the actual future in W*. But W and W* both have a future, and when we move from W to W* that--i.e., the future--changes.

Mike Almeida said...

One other quick point. The alternative assumes some form of essentialism about the future, but we don't assmue essentialism even about the past. Time travelers seem able to change the past or cause the past to be different, or else we can tell a coherent story on which they do. But then we'd certainly want to say that time travelers can change the future. If that's so, then essentialism about the future is false.