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Freedom and Time Travel

Ryan Wasserman Western Washington University

Time travel raises a number of interesting questions about the nature and possibility of freedom. This chapter introduces one of the most famous paradoxes of time travel (section 1) and outlines the standard response to that puzzle (section 2). It then considers the implications of these issues for the doctrine of fatalism (section 3), the analysis of freedom (section 4), and the debate over compatibilism (section 5).1

1. The Grandfather Paradox The Grandfather Paradox is one of the oldest and most familiar puzzles of time travel.2 There are many different versions of the paradox, but the most famous example in the philosophical literature is due to David Lewis:

Tim… detests his grandfather, whose success in the munitions trade built the family fortune that paid for Tim’s time machine. Tim would like nothing so much as to kill Grandfather, but alas he is too late. Grandfather died in his bed in 1957, while Tim was a young

* The published version of this paper will appear in J. Campbell, ed., A Companion to Free Will. Hoboken, NJ: Wiley. 1 For an alternative introduction to some of these topics, see Tognazzini (2016). 2 The grandfather paradox goes back to at least 1929 (see Gernsback (1929)), alt-hough related puzzles date back even farther (see, for example, Gaspar (1887)). For more on the history of the paradox, see Wasserman (2017b: Chapter 1 and Chapter 3, section 1).

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boy. But when Tim has built his time machine and traveled to 1920, suddenly he realizes that he is not too late after all. He buys a rifle; he spends long hours in target practice; he shadows Grandfather to learn the route of his daily walk to the munitions works; he rents a room along the route; and there he lurks, one winter day in 1921, ri-fle loaded, hate in his heart, as Grandfather walks closer, closer… (1976: 149)

According to Lewis, there is a good reason for thinking that Tim can kill Grandfather in the envisioned scenario. After all:

He has what it takes. Conditions are perfect in every way: the best rifle money could buy, Grandfather an easy target only twenty yards away, not a breeze, door securely locked against intruders. Tim a good shot to begin with and now at the peak of training, and so on… In short, Tim is as much able to kill Grandfather as anyone ever is to kill anyone. (1976: 149)3 However, there seems to be an equally good reason for thinking that

Tim cannot kill Grandfather. After all, if Grandfather were killed, then Father would not be born. And if Father were not born, then Tim would not be born either. But, in that case, Tim would not be able to travel back in time to kill Grandfather. Hence, time travel seems to imply the contra-diction that Tim both can and cannot kill Grandfather.4 More generally, time travel would seem to allow for people to perform “self-defeating ac-tions”—that is, actions whose performance would prevent the performance of the very act in question. Since it is impossible to perform this kind of action, it seems as if time travel must also be impossible. We will refer to this as the self-defeating problem for time travel.5

3 Lewis gives another argument for the same conclusion. For discussion of this argu-

ment, see Wasserman (2017a). 4 We will be focusing on the case of backward time travel (rather than forward time travel), so we will leave the ‘backward’ implicit from this point onward. 5 For a discussion of various kinds of self-defeating acts, see Wasserman (2017b: Chapter 3, section 1).

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One of the most common replies to this problem is to invoke a “branch-ing timeline” model of time travel, like the one depicted in Back to the Future II, Terminator Genisys, and Star Trek (2009).6 On this view, when Tim travels back to the past, he ends up in an alternative version of 1920 where he is free to kill Grandfather. This would make it the case that Tim is not born on the alternative timeline, but it would not change the fact that he was born on the original timeline. For this reason, Tim’s killing Grandfather would not be a self-defeating act. Unfortunately, the branching model of time travel is inconsistent with almost all of the traditional philosophical views about the nature of time. On the traditional way of thinking about it, there is a single 1920, and it either includes a killing of Grandfather or does not. Either way, that fact is already settled and there is no way to change it now.7 If this view about the mutability of the past is correct, then there is a second reason for thinking that Tim cannot kill Grandfather. Here is how Lewis puts the point:

Grandfather lived, so to kill him would be to change the past. But the events of a past moment… cannot change. Either the events of 1921 timelessly do include Tim’s killing of Grandfather, or else they timelessly don’t… It is logically impossible that Tim should change the past by killing Grandfather in 1921. So Tim cannot kill Grandfa-ther. (1976: 149)

This suggests a more general problem for the possibility of time travel: If time travel were possible, it would be possible to change the past. Since it is not possible to change the past, it is not possible to travel back in time. According to Lewis, this is the central problem raised by the grandfather paradox.8 9

6 The branching timeline view is closely related to the “multiverse” model of time travel that has been suggested by contemporary physicists (see, for example, Deutsch (1991)). For a discussion of these views, see Wasserman (2017b: Chapter 3, section 3). 7 See, for example, Cook (1982: 49), Horwich (1975: 436), and Hospers (1967: 177). 8 Note that this way of putting the problem focuses on modal claims about what an agent “can” or “cannot” do. But the same point can also be put in terms of freedom: If time travel were possible, then one would be free to change the past. But changing the

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2. The Standard Response

Generally speaking, there are three different ways of responding to the traditional grandfather paradox. First, one could accept the force of the argument and conclude that time travel is, in fact, impossible.10 Second, one could hold onto the possibility of time travel by challenging the claim that the past cannot be changed.11 Finally, one could resist the argument by insisting that—contrary to appearances—time travel would not put one in a position to change the past.12 Various ways of developing this final idea are possible, but the most familiar suggestion is due to David Lewis (1976).13 Let’s begin by thinking about a very different kind of case.14 Suppose that Anna is a competent English speaker (and reader) who would like nothing more than to spend the afternoon with Jane Austen’s Emma. Un-fortunately, someone has stolen the only available copy. Is it the case that Anna can still read Emma? In one sense, the answer is yes. After all, Anna is a competent reader of English, and Emma is the kind of book that a competent English reader can read. However, there is another sense in which Anna cannot read Emma since she lacks the opportunity to exercise that skill. As some might put it, she has the “general” ability to read Emma, but lacks the “specific” ability to read it in her current situation.15

past is logically impossible, and no one could ever be free to do what is logically impossi-ble. Hence, it is not possible to travel back in time. 9 For further discussion of this “past-alteration” problem, see Wasserman (2017: Chapter 3, section 2). For more on the relationship between the problem of self-defeat and the problem of past-alteration, see Sider (2002), Vihvelin (2011b), and Wasserman (2017: Chapter 4, section 2). 10 This seems to be the conclusion favored by Grey (1999: section 10). 11 This is one way of understanding the branching timeline model from the previous section. 12 See Vihvelin (1996). 13 Similar suggestions have been made by Thom (1975), Horwich (1975: 435-7), and Fitzgerald (1974), among others. 14 See Austin (1956). 15 See Honoré (1964), van Iwagen (1983: 13), and Mele (2002). For more on this distinction in the context of time travel, see Vihvelin (2011b).

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Lewis takes this kind of case to show that ‘can’ is context-sensitive. On his view, to say that someone can do something is to say that her perform-ing that action is compatible with all of the relevant facts. Since different facts are relevant in different contexts, ‘can’ ends up meaning different things.16 Here is one way of making this idea more precise. First, we analyze

‘can’-claims in terms of possibility. So, for example, we analyze (1) in terms of (2):

(1) Anna can read Emma.

(2) Possibly, Anna reads Emma. Second, we understand possibility claims as generalizations about possible worlds. (2), for example, is understood as follows: (3) ∃x (x is a possible world & Anna reads Emma in x) Finally, we take context to provide a set of facts that limit the domain of quantification by restricting it to those worlds at which the relevant facts hold. This is what Angelika Kratzer (2012) calls “the modal base”. For example, in some contexts, we might wonder whether Anna has the intrin-sic features required to read Emma (Are her eyes working? Does she under-stand English?) In those contexts, an utterance of (1) might express some-thing like the following: (4) ∃x (x is a possible world & Anna has the same intrinsic proper-

ties in x that she has in the actual world & Anna reads Emma in x)

This statement is obviously true—there are possible worlds in which Anna has her actual intrinsic properties and where she successfully reads Emma (just imagine, for example, a world that is like the actual world, but where her book is not stolen). Suppose instead that we are curious whether Anna has the opportunity to read Emma in her current situation (for example, we 16 On at least one meaning of ‘meaning’.

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might wonder whether she will be able to complete her reading assignment for class). In that case, (1) would express something like the following:

(5) ∃x (x is a possible world & Anna has the same intrinsic proper-ties in x that she has in the actual world & Anna’s local envi-ronment in x is just like her environment in the actual world & Anna reads Emma in x)

Unlike (4), this statement is false—if Anna’s environment in another possi-ble world is just like her environment in the actual world, there won’t be any copies of Emma around. So, Anna won’t be reading that book. Lewis takes this kind of context-sensitivity to be the driving force be-hind the grandfather paradox. Note, first, that the possibility of time travel would mean that there are some possible worlds in which Tim goes back in time and kills Grandfather. Some of these worlds are ones in which Grand-father is not really Tim’s grandfather (but where Tim mistakenly thinks that he is). Others are worlds in which Tim kills Grandfather, but where Grandfather miraculously rises from the grave (in order to sire Father, who will go on to sire Tim). Presumably, some of these worlds are ones in which Tim’s intrinsic properties and local environment are just like they are in the actual world (Tim has the same evil desires, the same make of gun, etc.). This provides a perfectly good sense in which Tim can kill Grandfather: his doing so is compatible with all of the facts about Tim’s intrinsic properties and local environment. However, on Lewis’s view there is also a perfectly good sense in which Tim cannot Grandfather. Suppose, for example, that we enrich the modal base by adding the fact that Grandfather was not actually killed in 1921. In that case, an utterance of ‘Tim can kill Grandfather’ would express the proposition that there is some possible world in which (a) Grandfather was not killed and (b) Tim kills Grandfather. Since there is no such world, the sentence would express a falsehood (relative to that context). The crucial point, for Lewis, is that there is no sense in which Tim both can and cannot kill Grandfather. That is what would be required in order for it to be the case that Tim can change the past. Hence, it is not possible to change the past, even if time travel is a reality.

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Here is how Lewis summarizes these points: Tim’s killing Grandfather that day in 1921 is compossible with a fairly rich set of facts: the facts about his rifle, his skill and train-ing… and so on… But his killing Grandfather is not compossible with another, more inclusive set of facts. [For example] There is the sim-ple fact that Grandfather was not killed… You can reasonably choose the narrower delineation, and say that he can; or the wider delinea-tion, and say that he can’t. But choose. What you musn’t do is wa-ver, say in the same breath that he both can and can’t, and then claim that this contradiction proves that time travel is impossible. (1976: 150-1)

The same point can be put in terms of “freedom”. On Lewis’s view, there is a sense in which Tim is free to kill Grandfather and a sense in which he is not. But there is no sense in which Tim is both free to kill Grandfather and not, so there is no contradiction. As a result, the possibility of time travel does not raise any new problems about human freedom. This is the stand-ard response to the grandfather paradox.17

3. Time Travel and Fatalism There is much to say in favor of the standard response. It is general in the sense that it provides a uniform solution to both the problem of self-defeat and the problem of past-alteration. It is conservative in the sense that it does not require a commitment to any controversial views about the nature of time (like the branching timeline model). And it is independently moti-vated in the sense that Lewis’s solution to the grandfather paradox can be applied to other problems as well. Consider, for example, the philosophical doctrine of fatalism. Fatalism is the view that whatever happens is una-voidable. On the assumption that freedom requires the ability to do other-

17 This response has been endorsed by many, including Sider (1997: 143), Smith (1997: 366), and Hanley (1997: 209-10).

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wise (i.e., the ability to avoid the relevant action) fatalism would imply that none of us is ever free.

Various arguments for fatalism have been put forward, but one of the oldest arguments begins with the principle of bivalence. According to this principle, every meaningful statement is either true or false, including statements about the future. Consider, for example, the statement that Ben will play with his Legos tomorrow. Suppose for the sake of argument that this statement is true. In that case, there is no way for anyone to prevent Ben from playing with his Legos tomorrow (including Ben!). After all, doing so would require making it false that he will play with his Legos tomorrow, and we have just assumed that this statement is true. Suppose instead that it is false that Ben will play with his Legos tomorrow. In that case, there is nothing anyone can do to bring it about that he plays with his Legos to-morrow, since doing so would require making that statement true. Once again, this would contradict our current assumption, since we have just taken it for granted that this statement is false. Given all of this, it follows that whatever happens tomorrow—Legos or no Legos—is unavoidable. Moreover, there is nothing special about Ben and his Legos—what goes for this case goes for every other future event. In other words, everything is unavoidable. This line of reasoning is often referred to as “the logical argu-ment” for fatalism.18 Most philosophers agree that there is something wrong with this argu-ment, but there is some disagreement over the details.19 According to Lewis, the problem with this argument is very similar to the problem with the grandfather paradox. When the Fatalist invites us to assume that Ben will play with his Legos tomorrow, what he is actually doing is encouraging us to add the statement that Ben will play with his Legos tomorrow to the modal base (along with, say, the facts about Ben’s properties and environ-ment). Given that, an utterance of ‘Ben can avoid playing with his Legos tomorrow’ would express something like the following:

18 This kind of argument is suggested by Aristotle in the ninth chapter of De Inter-pretatione. For a more recent articulation and defense of this argument, see Taylor (1962). 19 For some different diagnoses, see Prior (1967: 128-9), Purtill (1988), and Conee (in Conee and Sider 2005: 29-30).

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(6) ∃x (x is a possible world & Ben has the same intrinsic proper-

ties in x that he has in the actual world & Ben’s local environ-ment in x is just like his environment in the actual world & Ben plays with his Legos tomorrow in x & Ben avoids playing with his Legos tomorrow)

Since there is no such world, the sentence ‘Ben can avoid playing with his Legos tomorrow’ will express a falsehood, relative to that context. Similarly, when we are holding fixed the fact that Ben will not play with his Legos (assuming this is a fact), the sentence ‘Ben can avoid not playing with his Legos tomorrow’ will express something like the following:

(7) ∃x (x is a possible world & Ben has the same intrinsic proper-ties in x that he has in the actual world & Ben’s local environ-ment in x is just like his environment in the actual world & Ben does not play with his Legos tomorrow in x & Ben avoids not playing with his Legos tomorrow in x)

Once again, this statement is false. So, the sentence ‘Ben can avoid not playing with his Legos tomorrow’ will express a falsehood, relative to that context. The problem, however, is that this context is very different from the previous context, since the Fatalist has changed the modal base. The same thing is true for the conclusion of the argument, that whatever hap-pens—Legos or no Legos—is unavoidable. When the Fatalist asserts this conclusion, he is clearly leaving behind the assumption that that Ben will play with Legos and the assumption that he won’t. But, once we remove these statements from the modal base, the sentence ‘Ben either can’t avoid playing with Legos tomorrow or he can’t avoid not playing with Legos tomorrow’ will express something like the following:

(8) ~∃x (x is a possible world & Ben has the same intrinsic proper-ties in x that he has in the actual world & Ben’s local environ-ment in x is just like his environment in the actual world & Ben avoids playing with his Legos tomorrow in x) ∨ ~∃x (x is a

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possible world & Ben has the same intrinsic properties in x that he has in the actual world & Ben’s local environment in x is just like his environment in the actual world & Ben avoids not playing with his Legos tomorrow in x)

This statement is intuitively false—presumably, there are many possible worlds in which Ben (with his actual intrinsic properties and local environ-ment) plays Legos and many in which he does not. In other words, the future is not unavoidable. On this way of looking at things, the logical argument for fatalism is a bit like the following argument:

That is red. That is green. So, that is both red and green.

On the surface, this argument appears valid. But suppose that I am point-ing at a red apple when I assert the first premise, a green apple when I assert the second premise, and a yellow banana when I assert the conclu-sion. In that case, the argument will obviously be invalid, since ‘that’ will pick out different objects in each case. For Lewis, the same thing is true of the logical argument for fatalism—the word ‘can’ essentially picks out a different class of worlds in each case, owing to a change in context (in par-ticular, a change as to which facts are being held fixed). As a result, the Fatalist’s argument is invalid. Here is another way of putting Lewis’s point: When we are talking about what someone can or cannot do, we normally hold fixed certain facts about them and their surroundings. However, we do not normally hold fixed facts about their future—in particular, we do not normally hold fixed facts about what actions that agent will perform. For this reason, when we say that someone can avoid doing this or that, we normally speak the truth. Of course, there are possible contexts—“fatalistic contexts”, we might call them—in which we build facts about the future into the modal base. Rela-tive to those contexts, it will turn out that everything is unavoidable. How-

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ever, those contexts are not relevant when it comes to our everyday claims about what people can and cannot do. According to Lewis, the same thing is true of Tim and Grandfather. Given what we normally mean by ‘can’, Tim can kill Grandfather. Howev-er, because we know that Grandfather will not actually die, it is tempting to build this fact into the modal base. That is why many of us are tempted to say that Tim cannot kill Grandfather. But, as long as we resist this fatalistic tendency, we will not run into any problems: Given our ordinary standards, Tim can kill Grandfather.

4. Time Travel and Counterfactuals Lewis’s views on time travel and fatalism have proven very popular. How-ever, they are not without controversy. The most famous objection to his account is due to Kadri Vihvelin (1996, 2011a, 2011b). Vihvelin’s objection is based on the following principle about abilities and counterfactuals:

The Counterfactual Principle: Necessarily, if someone would fail to do something, no matter how hard or how many times she tried, then she cannot do it.

Whether or not this principle is correct will depend, in part, on what we mean by ‘can’. For example, we earlier noted a sense in which someone who is fluent in English can read Jane Austen’s Emma, even if there are no copies around. Clearly, that is not the sense of ‘can’ that Vihvelin has in mind. (After all, that reader would be sure to fail if she set out to read the book without any copies around.) Presumably, Vihvelin has in mind what we earlier called “specific” abilities—that is, abilities to perform actions in specific set of circumstances.20

20 Vihvelin (2011a) refers to these as “wide” abilities.

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Thus understood, the counterfactual principle seems entirely plausible.21 For one thing, many examples seem to conform to this pattern.

I can swim and ride a bicycle at least partly because it’s true that I would succeed at both of these activities if I tried to do them… But I cannot walk on water or run faster than the speed of light because if I tried to do these things, I would fail, no matter how often or how hard I tried. (1996: 318)

Moreover, this principle seems to underlie much of our practical reasoning about what to do:

Think about how you deliberate when you decide what to do. You choose among possible course of actions that you believe are your op-tions… But everyone agrees that a course of action is an option for you (something you have the wide ability to do) only if you would have some reasonable chance of doing it, if you tried, then and there, to do it. If the things you deliberate about are all things such that if you tried to do them, you would fail, you are wasting your time. (2011b) The problem, of course, is that all of this is inconsistent with what Lew-

is wants to say in the case of Tim and Grandfather. Grandfather must live in order for Tim to be present at all. So, we know that: (9) If Tim were to try to kill Grandfather, he would fail (no matter

how hard or how many times he tried). But then, given The Counterfactual Principle, we can infer: (10) Tim cannot kill Grandfather.

21 For discussion of The Counterfactual Principle, see Sider (2002), Vranas (2010), and Wasserman (2017b: Chapter 4, section 2).

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More carefully: Given that The Counterfactual Principle expresses a truth given what we ordinarily mean by ‘can’, (10) will express a truth, relative to ordinary contexts. This, however, is inconsistent with what Lewis wants to say about this case—according to him, Tim can kill Grandfather in the ordinary sense of ‘can’. If Vihvelin is correct, this claim is mistaken.

In order to appreciate Vihvelin’s view, it is important to be clear about what she does and does not say.

First, in disagreeing with Lewis about (10), she does reject the other parts of his theory. It is perfectly consistent with everything Vihvelin says to think that ‘can’-claims are restricted possibility claims, for example, and to think that they are sensitive to context in the way that Lewis suggests. The only disagreement between Lewis and Vihvelin is about whether or not Tim’s killing of Grandfather is compatible with the kinds of facts we nor-mally hold fixed when determining what someone can or cannot do.

Second, in disagreeing with Lewis, Vihvelin is not saying that the grand-father paradox is sound. In fact, she explicitly rejects this argument.22 Her view is that the very first premise of this argument is false—time travel is possible (she says) but that does not mean that Tim has the ability to kill Grandfather. Time travelers are simply limited in ways that ordinary indi-viduals are not. Third, in saying that Tim cannot kill Grandfather (in the ordinary sense of ‘can’), Vihvelin is not saying that this action is metaphysically impossi-ble. In fact, she explicitly says the opposite—for example, she says that there are worlds in which Tim travels back in time and kills Grandfather, only to see Grandfather miraculously rise from the grave (in order to sire Father, who will go on to sire Tim).23 Vihvelin’s point is that such a world would require very different laws of nature from our own, and is thus irrele-vant to evaluating ordinary counterfactuals like (9).24 Here it may be helpful to consider an analogy. Suppose, for the sake of argument, that there are some possible worlds in which I walk on water.

22 Vihvelin (1996: 323). 23 Vihvelin (1996: 317). 24 For a more complete explanation of this point (including a more general discussion of how laws of nature figure into the truth-conditions for counterfactuals) see Vihvelin (2011b) and Wasserman (2017b: Chapter 4, sections 2.1 and 2.3).

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Presumably, these are worlds with very different laws of nature than the actual world. For this reason, those worlds are irrelevant to the question of whether I would walk on water, were I to try. Intuitively, I would fail to walk on water no matter how hard or how many times I tried. So, The Counterfactual Principle implies (correctly) that I cannot walk on water. According to Vihvelin, the same thing is true for Tim: There might be some possible worlds in which Tim successfully kills Grandfather, but those worlds are not relevant to the question of whether or not Tim can kill Grandfather (in the ordinary sense of ‘can’). Interestingly, Vihvelin has a very different view about other acts of past-alteration. Suppose, for example, that Grandfather was neither killed nor pinched during 1921. And suppose that Tim goes back in time to shoot Grandfather, but has a last second change of heart. Could Tim then choose to pinch Grandfather instead? Vihvelin’s answer is “yes”. Presumably, Tim will not try to pinch Grandfather. But that does not mean he cannot. After all, there are many things we can do but won’t. What would have happened if Tim had tried to pinch Grandfather? Presumably, he would (or at least might) have succeeded. The complete explanation for this is complicated,25 but the short story is that pinching, unlike killing, would not be self-defeating. The only way for Tim to successfully kill Grandfather in the past would be if Grandfather were later resurrected from the dead (or if some other such miracle occurred). That is because Grandfather’s continued existence is required for Tim’s existence in the past. The same thing is not true when it comes to pinching. If Tim would have pinched Grandfather, there would not have been any large-scale miracles or other surprising events—Grandfather would simply have gone on to sire Father, who would have gone on to sire Tim, who would have gone back in time and pinched Grandfather. For this reason, Vihvelin claims that time travelers are free to perform many actions that they do not actually perform in the past. The only requirement is that these actions not be self-defeating.

5. Time Travel and Compatibilism 25 See Vihvelin (2011b) and Wasserman (2017b: Chapter 4, section 2.1 and 2.3).

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A different, and more general objection to Lewis is suggested by Michael Rea (2015). Like Vihvelin, Rea claims that time travelers are not free to perform self-defeating acts. However, he goes further than Vihvelin in claim-ing that time travelers are not free with respect to any of their actions in the past.26 Rea’s argument is based on two definitions, an observation, and a prin-ciple. The two definitions are as follows: (D1) e1 is a part of x’s causal history at e1 =def e1 stands in the an-

cestral of the causal relation to an event e2 that occurs at t and involves x as subject.

(D2) the causal history of x at t =def the sum of all events e such

that, at t, e is a part of x’s causal history. To say that e1 stands in the ancestral of the causal relation to e2 is to say that there is a series of events, a, b, c, … such that e1 is a cause of a, which is cause of b, which is cause of c, which… is a cause of e2. So, for example, my choice of college played a role in my studying philosophy, which played a role in my writing this paper. So, my choice of college is a part of my causal history, relative to the current time. And, of course, my causal histo-ry will go back much further than this—from my schooling to my birth to my parents’ meeting and all the way back, ultimately, to the beginning of the universe. Rea next observes that, for time travelers, events from one’s causal history can fall in one’s temporal future. This might sound paradoxical at first, but it is exactly what we should expect in the case of backward time travel. After all, in that kind of case, the traveler’s departure is a cause of his arrival, even though the arrival precedes the departure in time. Indeed, this kind of mismatch between causal history and temporal history is often

26 Indeed, Rea goes even further than this, arguing that the lack of time traveler freedom transfers to non-time-travelers as well, so that “time travel destroys freedom on a global scale.” (2015: 279)

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said to be definitive of backward time travel.27 But the main point is that a time traveler’s causal history can include future events.

Rea argues that this point leads to the conclusion that time travelers are not free. His argument is based on this principle:

The Causal History Principle: No one is able, at a time t or later, to refrain from an act that is part of her causal history at t, or to per-form an act such that, for some event e in her causal history, neces-sarily, she performs the act only if e is not in her causal history.

According to Rea, this principle embodies a traditional incompatibilist view on freedom—namely, the view that someone is only free to perform an action if it is compatible with all of the facts about the past and the laws of nature. Crucially, Rea takes “the past” in this case to refer to the causal history of the relevant individual (rather than the temporal history). This is what leads to trouble for time travellers. Consider, again, the case of Tim. Tim’s refraining from killing Grandfather (at t2, let’s say) is a part of his causal history at t1 (as he takes aim at Grandfather). So, given The Causal History Principle, Tim is unable to kill Grandfather, even though he has a loaded gun, a clear shot, etc. The same thing is true for many other actions. Suppose, for example, that Tim travels back to the days of his youth and gives his younger self a shiny new present (a present that he himself remembers receiving from a stranger when he was a child). Could Tim (the older Tim) have refrained from performing this action? Not if Rea is correct. After all, the giving the gift (at t2, let’s say) is a cause of the memory that (older) Tim has at t1 (as he approaches his younger self with the gift). So, it is a part of his causal history. But then, given The Causal History Principle, it follows that Tim is not able to do otherwise. There are various things that one might say in response to Rea, but Lewis and Vihvelin will presumably want to deny his Causal History Prin-ciple (along with the more general incompatibilist view that it embodies). Of course, they will grant that facts about an agent’s causal history are relevant to the question of what she will do in the future. Indeed, if the laws 27 See, for example, Lewis (1976: 146) and Wasserman (2017b: Chapter 1, section 1).

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are deterministic, then those facts will determine what she will do in the future. But those facts are not (directly) relevant to the question of what an agent can do in a given situation (not, at least, in the ordinary sense of ‘can’). If we want to know whether or not Anna is able read Emma, for example, we need to know about her and her environment. (Does she un-derstand English? Is there a copy of Emma around?) But we don’t need to know any further facts about her causal history. (Did she have parents? When did she go to school? What was the state of the universe like a billion years before she was born?) That is why we hold the first (but not the second) kind of facts fixed when evaluating the claim that Anna is able to read Emma. In other words, consistency with causal history is not a prereq-uisite for ability. That, at least, is what the compatibilist will claim.

6. Conclusion As this brief survey indicates, the possibility of time travel raises a number of interesting questions about freedom, fatalism, and compatibilism. It also intersects with more general debates about modality, counterfactuals, and the laws of nature. For these reasons, time travel has been a topic of much interest to philosophers, and will continue to be so for the foreseeable fu-ture.

References

Conee, Earl and Sider, Theodore. 2005. Riddles of Existence. Oxford: Ox-ford University Press.

Cook, Monte. 1982. Tips for Time Travel. In N. D. Smith, ed., Philosophers Look at Science Fiction. Chicago: Nelson-Hall.

Daniels, David R. 1935. The Branches of Time. Wonder Stories 7: 294-303. Deutsch, David. 1991. Quantum Mechanics near Closed Timelike Lines.

Physical Review D 44: 3197-3217. Fitzgerald, Paul. 1974. On Retrocausality. Philosophia 4: 513-51.

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Gaspar, Enrique. 1887/2012. The Time Ship A Chrononautical Journey, translated by Yolanda Molina-Gavilán and Andrea Bell. Middletown, CT: Wesleyan University Press.

Gernsback, Hugo. 1929. The Question of Time-Traveling. Science Wonder Stories vol.1, #7.

Hanley, Richard. 1997. The Metaphysics of Star Trek. New York: Basic Books.

Honoré, A.M. 1964. Can and Can’t. Mind 73: 463–479. Horwich, Paul. 1975. On Some Alleged Paradoxes of Time Travel. The

Journal of Philosophy 72: 432-444. Hospers, John. 1967. An Introduction to Philosophical Analysis. London:

Routledge & K. Paul Kratzer, Angelika. 2012. Modals and Conditionals. Oxford: Oxford Universi-

ty Press. Lewis, David. 1976. The Paradoxes of Time Travel. American Philosophical Quarterly 13: 145-152.

Mele, Alfred. 2002. Agents’ Abilities. Noûs 37: 447–470. Prior, A.N. 1967. Past, Present and Future. Oxford: Oxford University Press.

Purtill, Richard. 1988. Fatalism and the Omnitemporality of Truth. Faith and Philosophy 5: 185–192.

Rea, Michael. 2015. Time Travelers are not Free. Journal of Philosophy 112: 266-279

Sider, Theodore. 1997. A New Grandfather Paradox? Philosophy and Phe-nomenological Research 57: 139-144

___ 2002. Time Travel, Coincidences and Counterfactuals. Philosophical Studies 110: 115–138.

Smith, Nicholas J. J. 1997. Bananas Enough for Time Travel? British Jour-nal for the Philosophy of Science 48: 363-389.

Taylor, Richard. 1962. Fatalism. Philosophical Review 71: 56-66. Thom, Paul. 1975. Time Travel and Non-Fatal Suicide. Philosophical Stud-ies 27: 211-216.

Tognazzini, Neal. 2016. Free Will and Time Travel. In M. Griffith, N. Levy, and K. Timpe, eds., The Routledge Companion to Free Will. New York: Routledge.

19

van Inwagen, Peter. 1983. An Essay on Free Will. Oxford: Oxford Universi-ty Press.

Vihvelin, Kadri. 1996. What Time Travelers Cannot Do. Philosophical Studies 81: 315-330.

___ 2011a. Ability, ‘Can’, and Counterfactuals. Vihvelin.com. ___ 2011b. Counterfactuals, Indicatives and What Time Travelers Can’t

Do. Vihvelin.com. Vranas, Peter. 2010. What Time Travelers May be Able to Do. Philosophi-

cal Studies 150: 115–21. Wasserman, Ryan. 2017a. Time Travel, Ability, and Arguments by Analo-

gy. Thought 6: 17-23. ___ 2017b. Paradoxes of Time Travel. Oxford: Oxford University Press.