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Ability, ‘Can’, and Counterfactuals

I think that time travel is logically possible, but I think that even if it were physically possible no time traveler would be able to go back in time and kill her baby self. My argument has two premises:

Premise1: A person can do something (is able to do that thing) only if it isn't always true that: if he tried to do it, he would fail.

Premise 2: It is always true of Suzy, the time traveller, that if she tried to kill the baby who is her younger self, she would fail.

In this post I'm going to defend the first premise. That premise isn't specific to time travelers. It asserts a quite general link between our beliefs about what a person can do and the truth of certain counterfactuals. That is why (1) is worth thinking about quite apart from worries about time travel: it captures an essential feature of what we mean when we say that people are able to do things even if they don't do them. It  partly defines how we understand 'can' when we make choices: that is, when we choose to do one thing from among the other things we can do.

What TimeTravelers Cannot Do

This is first of a series of posts on TimeTravel.

I'm interested in the philosophically interesting kind of time travel, by which I mean time travel to the one and only actual past, and, in particular, to your past, to the time when your parents were children, or to the time when you were a baby. I won't be talking about time travel to a parallel universe or to a different dimension of time. I will also be limiting my attention to cases of time travel at possible worlds that are as much like ours as is consistent with time travel; so, unless there is reason to suppose otherwise, I will be assuming that a time travel world is governed by laws much like our own laws.

I believe that this kind of time travel is logically and metaphysically possible.

This is now the majority view, at least among those who think and write about time travel . So this part of my view is not especially interesting.

However, I depart from the other defenders of time travel in insisting that time travel, while possible, is very odd. David Lewis, whose 1976 paper, "The Paradoxes of Time Travel", remains the classic defense of time travel, said that time travel is possible but also cautioned us that "a world where time travel took place would be a most strange world, different in fundamental ways from the world we think is ours". Lewis's paper succeeded in convincing many philosophers that time travel is possible. Indeed, he succeeded so well that some of the contemporary defenders of time travel argue that time travel isn't merely possible, it isn't that strange either.

I disagree. Time travel is possible, but I insist that it is also strange. Time travel worlds are backwards causation worlds – worlds where some causes happen after their effects – and this makes a huge difference to our common-sense view of the world and ourselves.

These posts are about one of the ways in which time travel is strange. I will be arguing that if our world turns out to be a time travel world, then we are not entitled to some of our common-sense assumptions about our abilities. If our world is a time travel world, then we have some abilities that common-sense says we do not have. And if our world is a time travel world, then we lack some abilities that common-sense says we have. But before I get to this, I want to point out another way in which time travel is very strange.

The Consequence Argument and Lewis's Reply

The only serious argument for incompatibilism that I know is the Consequence argument due, most famously, to Peter van Inwagen.  (An Essay on Free Will, OUP, 1983.)

The version I will discuss is due to David Lewis.  ("Are We Free to Break the Laws?”, Theoria 47 (1981), 113-121)

He tells us to think of the argument as a reductio.  A  compatibilist is someone who claims that the truth of determinism is compatible with the existence of the kinds of abilities that we assume we have in typical situations in which we deliberate and make a choice.   Let’s call these ‘ordinary abilities’. The Consequence argument claims that if we suppose that a deterministic agent has ordinary abilities, we are forced to credit her with incredible abilities as well.

Here is Lewis's argument.

Pretend that  determinism is true, and that I did not raise my hand (at that department meeting, to vote on a proposal) but had the ordinary ability to do so.  If I had exercised my ordinary ability – if I had raised my hand -- then either the remote past or the laws of physics would have been different (would have to have been different). But if that’s so, then I have at least one of two incredible abilities – the ability to change the remote past or the ability to change the laws. But to suppose that I have either of these incredible abilities is absurd.  So we must reject the claim that I had the ordinary ability to raise my hand.

Van Inwagen doesn't object to Lewis's way of stating his argument.  On the contrary, he has said that Lewis's paper  is “the finest essay that has ever been written in defense of compatibilism – possibly the finest essay that has ever been written about any aspect of the free will problem”.  ("How to Think about the Problem of Free Will”, Journal of Ethics (2008) 12, 337-341).

Van Inwagen now agrees that the Consequence argument fails as a reductio.

However, he claims that it has nevertheless succeeded in  "raising the price" of compatibilism.  (Freedom to Break the Laws", Midwest Studies in Philosophy, 28 (2004), Blackwell, 334-350).

I disagree. I say that the argument neither succeeds as a reductio nor succeeds in  "raising the price" of compatibilism - that is, the price of commonsense  at a deterministic world.  What the argument does achieve -- at least on Lewis's articulation of it -- is a clear statement of the counterfactuals to which the compatibilist is committed. The argument is valuable for this reason.  It makes it clear that we need to understand counterfactuals in order to understand what's at stake in the free will/determinism debate. But as an argument for incompatibilism, it fails.