If time travel is possible, presumably so is my shooting my younger self ; then apparently I can kill him – I can commit retrosuicide. But if I were to kill him I would not exist to shoot him, so how can I kill him? The standard solution to this paradox understands ability as compossibility with the relevant facts and points to an equivocation about which facts are relevant: my killing YS is compossible with his proximity but not with his (...) survival, so I can kill him if facts like his survival are irrelevant but I cannot if they are relevant. I identify a lacuna in this solution, namely its reliance without argument on the hidden assumption that my killing YS is possible: if it is impossible, it is not compossible with anything. I argue that this lacuna is important, and I sketch a different solution to the paradox. (shrink)

This paper addresses a worry about backwards time travel. The worry is that there is something mysteriously inexplicable about the combination of commonplace events that will inevitably conspire to prevent the time traveler from doing something impossible such as killing her younger self. The worry is first distinguished from other problems for backwards time travel concerning its alleged impossibility or improbability. It is then shown that the worry is misplaced: there is in fact no real problem here. Yet the worry (...) has been widely expressed—so a suggestion is also made as to why it is so easy to get into the position of thinking that there is a genuine problem here, when in fact there is not. Finally, in light of the resolution of the inexplicability worry, a new way of dealing with the other two problems for backwards time travel—concerning its alleged impossibility and improbability—is proposed. (shrink)

Whereas many philosophers take backwards causation to be impossible, the few who maintain its possibility either take it to be absent from the actual world or else confined to theoretical physics. Here, however, I argue that backwards causation is not only actual, but common, though occurring in the context of our social institutions. After juxtaposing my cases with a few others in the literature and arguing that we should take seriously the reality of causal cases in these contexts, I consider (...) several objections. These objections involve whether the cases should be reinterpreted, whether they are properly within the institution, whether they involve necessitation or else Cambridge changes, whether and how they involve changing the past, and whether this should call us to question institutional reality. I end by suggesting that it is a virtue of our institutions that they allow backwards causation, that this is a kind of technology that they are often built to incorporate. (shrink)

In no possible world does a time traveler succeed in killing herearlier self before she ever enters a time machine. So if many,many time travelers went back in time trying to kill theirunprotected former selves, the time travelers would fail inmany strange, coincidental ways, slipping on bananapeels, killing the wrong victim, and so on. Such cases producedoubts about time travel. How could ``coincidences'' beguaranteed to happen? And wouldn't the certainty of coincidentalfailure imply that time travelers are not free to killtheir (...) earlier selves? But if so, what would inhibit theirfreedom? Despite initial appearances, these and other doubtsmay be answered: the possibility of time travel survives yetanother challenge. (shrink)

It is not uncommon in time travel stories to find that the mechanism by which the time travel is achieved is not invented. A time traveller could journey to his/her own past and give the designs of the time travel machine to his/her earlier self as s/he was given the designs as a younger person. These designs never get thought up by anyone. Such a situation would conflict with the usual conception of the acquisition of knowledge. This situation is called (...) the Temporal Epistemic Anomaly and would arise if knowledge is gained at a time prior to the information in question being transmitted but is not discovered or invented at any time. This article examines the implications of information propagating around a causal chain that is closed in time (which is required to create the Anomaly) and whether this information need have a specific origin point. (shrink)

David Lewis famously argued that the time traveller ‘can’ murder her grandfather, even though she never will: it is compossible with a particular set of facts including her motive, opportunity and skill . I argue that while ordinary agents ‘can’ under Lewis’s conception, philosophers cannot – the latter will not only fail to fulfill their homicidal intentions but also fail to form them in the first place.

Causal loops are a recurring feature in the philosophy of time travel, where it is generally agreed that they are logically possible but may come with a theoretical cost. This paper introduces an unfamiliar set of causal loop cases involving knowledge or beliefs about the future: self-fulfilling prophecy loops (SFP loops). I show how and when such loops arise and consider their relationship to more familiar causal loops.

Frege's scathing comments on Mill on the empirical grounds of arithmetical truth are elaborated. The suggestion is made that some entities are ‘well-behaved' : if you perform two acts and then two more, the ‘result' will be that exactly four acts have occurred. How much it all matters or means is not further discussed.

It continues to be alleged that superluminal in uences of any sort would be inconsistent with special relativity for the following three reasons: they would imply the existence of a ‘distinguished’ frame; they would allow the detection of absolute motion; and they would violate the relativity of simultaneity. This paper shows that the first two objections rest upon very elementary misunderstandings of Minkowski geometry and on lingering Newtonian intuitions about instantaneity. The third objection has a basis, but rather than invalidating (...) the notion of faster-than-light influences it points the way to more general conceptions of simultaneity that could allow for quantum nonlocality in a natural way. (shrink)

This paper argues for the following disjunction: either we do not live in a world with a branching temporal structure, or backwards time travel is nomologically impossible, given the initial state of the universe, or backwards time travel to our space-time location is impossible given large-scale facts about space and time. A fortiori, if backwards time travel to our location is possible, we do not live in a branching universe.

According to the No Alternate Possibilities argument, if time passes then the rate at which it passes could be different but time cannot pass at different rates, and hence time cannot pass. Typically, defenders of the NAP argument have focussed on defending premise, and have taken the truth of for granted: they accept the orthodox view of rate necessitarianism. In this paper we argue that the defender of the NAP argument needs to turn her attention to. We describe a series (...) of worlds that appear to contain differential passage: worlds where time passes at different rates in different subregions. If the NAP argument is to succeed, rate necessitarians must show that each of these worlds is either metaphysically impossible, or does not contain differential passage. (shrink)

As a discipline of its own, the philosophy of science can be traced back to the founding of its academic journals, some of which go back to the first half of the twentieth century. While the discipline has been the object of many historical studies, notably focusing on specific schools or major figures of the field, little work has focused on the journals themselves. Here, we investigate contemporary philosophy of science by means of computational text-mining approaches: we apply topic-modeling algorithms (...) to eight major philosophy of science journals, from the 1930s up until 2017. Based on the full-text content of some 15,897 articles, we identified 25 research themes and 8 thematic clusters that show how the research agenda of the philosophy of science has changed in its content over the course of the last eight decades, up to the philosophy of science we now know. We also show how each one of the journals contributed in its own way to this thematic evolution. (shrink)

Philosophers have long wondered whether God exists; and yet, they have ignored the question of whether we should hope that He exists – call this stance aspirational theism. In this article, I argue that we have a weighty pro tanto reason to adopt this stance: theism offers a metaphysical guarantee against gratuitous suffering. On the other hand, few atheist alternatives offer such a guarantee – and even then, there are reasons to worry that they are inferior to the theistic alternative. (...) Given this difference, we have a strong pro tanto, but not all-things-considered, reason to adopt aspirational theism. (shrink)

If there is a second dimension of time – a so-called ‘hypertime’ – is it logically possible for the past to change? Some have said yes; others have said no. I say yes provided that one has the appropriate ontological view of hypertime. So far, the ontology of hypertime has seldom been discussed. As such, this paper not only defends the logical possibility of a changing past, but aims to start a discussion on what ontological commitments are required to make (...) sense of a changing past. (shrink)

Time travel and its associated paradoxes are a topic of academic discussion that has historically been of interest only in physics and philosophy. This paper presents a new paradox of time travel that puts psychological issues at the forefront. The new time traveller faces conflicts between agency and agency awareness that are not addressed in other paradoxes. Further, in considering these other paradoxes it is seen how concerns about the time traveller's psychological state can lead to new challenges to the (...) accepted deterministic paradox resolutions. (shrink)

There have been many objections to the possibility oftime travel. But all the truly interesting ones concern the possibility of reversecausation. What is objectionable about reverse causation? I diagnose that the trulyinteresting objections are to a further possibility: that of causal loops. I raisedoubts about whether there must be causal loops if reverse causation obtains; but devote themajority of the paper to describing, and dispelling concerns about, various kinds ofcausal loop. In short, I argue that they are neither logically nor (...) physically impossible.The only possibly objectionable feature that all causal loops share is that coincidenceis required to explain them. Just how coincidental a loop will be varies: some arereally quite ordinary, and some are incredibly unlikely. I end by speculating thatthe tendency amongst physicists to avoid discussion of causal loops involving intentionalaction may have been unfortunate, since intentional action is an excellent way tonon-mysteriously bring about what otherwise would have been an unlikely coincidence. Hencecausal loops may be more likely in a world with beings like us, than in one without. (shrink)

Three arguments for the conclusion that objects cannot endure in B-time even if they remain intrinsically unchanged are examined: Carter and Hestevolds enduring-objects-as-universals argument (American Philosophical Quarterly 31(4):269-283, 1994) and Barker and Dowe's paradox 1 and paradox 2 (Analysis 63(2):106-114, 2003, Analysis 65(1):69-74, 2005). All three are shown to fail.

Despite a growing number of models argument for the logical possibility of changing the past there continues to be resistance to and confusion surrounding the possibility of changing the past. In this paper I shall attempt to mitigate the resistance and alleviate at least some of the confusion by distinguishing changing the past from what Richard Hanley calls ‘annulling’ the past and distinguishing both from what I shall call ‘otherwising’ the past.

A world in which time travel into the past occurred would seem like a most strange world. Nicholas Smith, however, in his ‘Bananas Enough for Time Travel’, argues that time travel is not so strange as we think. In particular, he argues against what he views as the main reason time travel worlds seem so strange – the claim that time travel entails unusual numbers of coincidences. I shall argue that Smith's argument for rejecting the claim is inadequate. Hence, the (...) debate over the consequences of the coincidences of time travel cannot be avoided by denying the coincidences themselves. (shrink)

A world in which time travel into the past occurred would seem like a most strange world. Nicholas Smith, however, in his ‘Bananas Enough for Time Travel’, argues that time travel is not so strange as we think. In particular, he argues against what he views as the main reason time travel worlds seem so strange – the claim that time travel entails unusual numbers of coincidences. I shall argue that Smith's argument for rejecting the claim is inadequate. Hence, the (...) debate over the consequences of the coincidences of time travel cannot be avoided by denying the coincidences themselves. (shrink)

We standardly evaluate counterfactuals and abilities in temporally asymmetric terms—by keeping the past fixed and holding the future open. Only future events depend counterfactually on what happens now. Past events do not. Conversely, past events are relevant to what abilities one has now in a way that future events are not. Lewis, Sider and others continue to evaluate counterfactuals and abilities in temporally asymmetric terms, even in cases of backwards time travel. I’ll argue that we need more temporally neutral methods. (...) The past shouldn’t always be held fixed, because backwards time travel requires backwards counterfactual dependence. Future events should sometimes be held fixed, because they’re in the causal history of the past, and agents have evidence of them independently of their decisions now. We need temporally neutral methods to maintain connections between causation, counterfactuals and evidence, and if counterfactuals are used to explain the temporal asymmetry of causation. (shrink)

Do time travellers retain their normal freedom and abilities when they travel back in time? Lewis, Horwich and Sider argue that they do. Time-travelling Tim can kill his young grandfather, his younger self, or whomever else he pleases—and so, it seems can reasonably deliberate about whether to do these things. He might not succeed. But he is still just as free as a non-time traveller. I’ll disagree. The freedom of time travellers is limited by a rational constraint. Tim can’t reasonably (...) deliberate on killing his grandfather, certain that he’ll fail. If Tim follows his evidence, and appropriately self-predicts, he will be certain he won’t kill his grandfather. So if Tim is both evidentially and deliberatively rational, he can’t deliberate on killing his grandfather. This result has consequences. Firstly, it shows how evidential limits in the actual world contribute to our conception of the future as open. Secondly, it undercuts arguments against the possibility of time travel. Thirdly, it affects how we evaluate counterfactuals in time travel worlds, as well as our own. I’ll use the constraint to motivate an evidential and temporally neutral method of evaluating counterfactuals that holds fixed what a relevant deliberating agent has evidence of, independently of her decision. Using this method, an agent’s local abilities may be affected by what happens globally at other times, including the future. (shrink)

A plausible thought is that we should evaluate counterfactuals in the actual world by holding the present ‘fixed’; the state of the counterfactual world at the time of the antecedent, outside the area of the antecedent, is required to match that of the actual world. When used to evaluate counterfactuals in the actual world, this requirement may produce reasonable results. However, the requirement is deeply problematic when used in the context of explaining causal asymmetry. The requirement plays a crucial role (...) in certain statistical mechanical explanations of the temporal asymmetry of causation. I will use a case of backwards time travel to show how the requirement enforces certain features of counterfactual structure a priori. For this reason, the requirement cannot be part of a completely general method of evaluating counterfactuals. More importantly, the way the requirement enforces features of counterfactual structure prevents counterfactual structure being derived from more fundamental physical structure—as explanations of causal asymmetry demand. Therefore, the requirement cannot be used when explaining causal asymmetry. To explain causal asymmetry, we need more temporally neutral methods for evaluating counterfactuals—those that produce the right results in cases involving backwards time travel, as well as in the actual world. (shrink)

What's the point of time travel? Not to change the past; no matter how carefully a time traveler plans, all of her attempts to change the past end in failure. Paul Horwich has argued that the implausibility of such failures gives us reason to doubt that there will be frequent time travel to the local past. I defend a modified version of Horwich's argument and show how we might gain evidence about the chance of there being frequent time travel in (...) the future without having any information that explains why that chance value obtains. (shrink)

This article discusses the possibility of some outlandish tropes from time travel fiction, such as people reversing in age as they time travel or the universe being destroyed because a time traveler kills their ancestor. First, I discuss what type of possibility we might have in mind, detailing ‘close possibility’ as one such candidate. Secondly, I argue that—with only little exception—these more outlandish tropes fail to be closely possible. Thirdly, I discuss whether these outlandish tropes may nevertheless be more broadly (...) possible (e.g., metaphysically or logically possible), arguing that whether they are or not depends upon your favored metaphysics of the laws of nature. (shrink)

This paper investigates ‘exterminous hypertime’, a model of time travel in which time travellers can change the past in virtue of there being two dimensions of time. This paper has three parts. Part one discusses the laws which might govern the connection between different ‘hypertimes’, showing that there are no problems with overdetermination. Part two examines a set of laws that mean changes to history take a period of hypertime to propagate through to the present. Those laws are of interest (...) because: (i) at such worlds, a particular problem for non-Ludovician time travel (‘the multiple time travellers’ problem) is avoided; and (ii) they allow us to make sense of certain fictional narratives. Part three discusses how to understand expectations and rational decision making in a world with two dimensions of time. I end with an appendix discussing how the different theories in the metaphysics of time (e.g., tensed/tenseless theories and presentism/eternalism/growing block theory) marry up with exterminous hypertime. (shrink)

In this paper I consider two objections raised by Nick Smith (1997) to an argument against the probability of time travel given by Paul Horwich (1995, 1987). Horwich argues that time travel leads to inexplicable and improbable coincidences. I argue that one of Smith's objections fails, but that another is correct. I also consider an instructive way to defend Horwich's argument against the second of Smith's objections, but show that it too fails. I conclude that unless there is something faulty (...) in the conception of explanation implicit in Horwich's argument, time travel presents us with nothing that is inexplicable. (shrink)

It is claimed that unacceptable constraints on initial data are imposed by certain responses to paradoxes that threaten time travel, closed timelike curves (CTCs) and other backwards causation hypotheses. In this paper I argue against the following claims: to say “contradictions are impossible so something must prevent the paradox” commits in general to constraints on initial data, that for fixed point dynamics so-called grey state solutions explain why contradictions do not arise, and the latter have been proved to avoid constraints (...) on initial data. †To contact the author, please write to: Philosophy, University of Queensland, Brisbane, Queensland 4072, Australia; e-mail: [email protected]. (shrink)

In Making it Explicit Robert Brandom claims that perspectivally hybrid de re attitude ascriptions explain what an agent actually did, from the point of view of the ascriber, whether or not that was what the agent intended to do. There is a well-known problem, however, first brought to attention by Quine, but curiously ignored by Brandom, that threatens to undermine the role of de re ascriptions in the explanation of action, a problem that stems directly from the fact that, unlike (...) de dicto ascriptions, they permit the attribution of inconsistent attitudes to agents. I propose a solution to the problem which I believe is consistent with Brandom’s approach to the nature of intentionality and the explanation of action. (shrink)

The philosophy of time travel is a sub-field of metaphysics – the study of what there is and what things are like – that considers questions about the possibility of time travel and what a world in which time travel is possible looks like. These questions range from whether time travel is actually possible, to how time travellers can act in the past or future. This thesis delves into a particularly interesting, yet historically undertreated theme: the abilities of time travellers (...) and relatedly their freedom as well. The abilities and freedom of time travellers has been the source of a lot of recent discussion with some polarising views. The minority (which includes Lewis (1976), but also Ted Sider (2002)) argue for the affirmative – that time travellers are free and have the same (or perhaps more) abilities as non-time travellers. The majority (which includes Kadri Vihvelin (1996), Michael Rea (2005), Stephanie Rennick (2015), Alison Fernandes (2020), amongst others) argue for the negative, that time travellers do not have exactly the same freedoms and abilities as non-time travellers. I think that it is a mistake to be so pessimistic about the abilities and freedom of time travellers. I treat concerns about the freedom of time travellers and concerns about the abilities of time travellers as two sides of the same coin. Therefore, in what follows, I argue for two related conclusions. First, I argue that time travellers have all sorts of abilities. Second, I argue that time travellers are just as free as non-time travellers. In arguing for these conclusions, I first produce a novel analysis of ability which reveals important details about how abilities track inputs rather than outputs. Second, I use this new analysis of ability to present a new compatibilist account of free will. If I am right in arguing positively for the abilities and freedoms of time travellers, then this has large implications for both the current time travel discourse and the general debate surrounding freedom. Significantly, I am arguing against some prominent views about time travel and in suggesting that these views are mistaken, I am presenting new ideas about ability and freedom: specifically, that time travellers have the ability and are free to do impossible things. (shrink)

A book review of R. Paul Thompson's "A Remarkable Journey: The Story of Evolution.

Tim travels back in time and tries to kill his grandfather before his father was born. Tim fails. But why? Lewis's response was to cite "coincidences": Tim is the unlucky subject of gun jammings, banana peels, sudden changes of heart, and so on. A number of challenges have been raised against Lewis's response. The latest of these focuses on explanation. This paper diagnoses the source of this new disgruntlement and offers an alternative explanation for Tim's failure, one that Lewis would (...) not have liked. The explanation is an obvious one but controversial, so it is defended against all the objections that can be mustered. (shrink)

The present paper advances an analogy between cases of extra-mathematical explanation and cases of what might be termed ‘extra-logical explanation’: the explanation of a physical fact by a logical fact. A particular case of extra-logical explanation is identified that arises in the philosophical literature on time travel. This instance of extra-logical explanation is subsequently shown to be of a piece with cases of extra-mathematical explanation. Using this analogy, we argue extra-mathematical explanation is part of a broader class of non-causal explanation. (...) This has important implications for extra-mathematical explanation, for time travel and for theories of explanation more generally. (shrink)

We argue that explanations appealing to logical impossibilities are genuine explanations. Our defense is based on a certain picture of impossibility. Namely, that there are impossibilities and that the impossibilities have structure. Assuming this broad picture of impossibility we defend the genuineness of explanations that appeal to logical impossibilities against three objections. First, that such explanations are at odds with the perceived conceptual connection between explanation and counterfactual dependence. Second, that there are no genuinely contrastive why-questions that involve logical impossibilities (...) and, third, that explanations appealing to logical impossibilities rule nothing out. (shrink)

The standard philosophical view of time travel has it that time travelers cannot change the past. It has been argued by some that the standard view is false, and that this can be shown using a two-dimensional model of time. I defend the standard view against this attack. I show, first, that the addition of a second temporal dimension does not provide a model of changing the past and, second, that neither does the addition of n temporal dimensions for any (...) n > 1. (shrink)

This paper addresses a particular interpretation of quantum mechanics, i.e. superdeterminism. In short, superdeterminism i) takes the world to be fundamentally deterministic, ii) postulates hidden variables, and iii) contra Bell, saves locality at the cost of violating the principle of statistical independence. Superdeterminism currently enjoys little support in the physics and philosophy communities. Many take it to posit the ubiquitous occurrence of hard-to-digest conspiratorial and coincidental events; others object that violating the principle of statistical independence implies the death of the (...) scientific methodology. In this paper, we offer a defense to these and other objections. To counter the conspiracy objection, we draw upon the philosophical literature on time travel, and conclude that the picture of the world offered by the superdeterminist does not need to be particularly surprising or conspiratorial. We then move on to other recent objections, in particular those that focus on the methodology of science and the nature of the physical laws compatible with superdeterminism. A key ingredient of our arguments is that the principle of statistical independence may be violated in theory, but valid for practical purposes. Our overarching goal is to offer a defense of superdeterminism with respect to its main objections, so that it can earn its keep as a legitimate contender among the possible interpretations of quantum mechanics. (shrink)

{June 2018 UPDATE: This work has been greatly surpassed by "Utilizing Future-Viewing Instruments" which will appear in the July 2018 issue of Progress in Physics. It can now be downloaded in PDF form from their website.} This is the full text of the paper that was published as a Kindle book on April 6th, 2014. In most respects, it has since been surpassed by "Potentials of Future-Viewing Machines," which is available here in this repository. Nevertheless, this earlier work addresses minutia (...) and interesting considerations that do not appear elsewhere, and it has a large bibliography, so it remains very useful for research purposes. (shrink)

This paper is an enquiry into the logical, metaphysical, and physical possibility of time travel understood in the sense of the existence of closed worldlines that can be traced out by physical objects. We argue that none of the purported paradoxes rule out time travel either on grounds of logic or metaphysics. More relevantly, modern spacetime theories such as general relativity seem to permit models that feature closed worldlines. We discuss, in the context of Gödel's infamous argument for the ideality (...) of time based on his eponymous spacetime, what this apparent physical possibility of time travel means. Furthermore, we review the recent literature on so-called time machines, i.e., of devices that produce closed worldlines where none would have existed otherwise. Finally, we investigate what the implications of the quantum behaviour of matter for the possibility of time travel might be and explicate in what sense time travel might be possible according to leading contenders for full quantum theories of gravity such as string theory and loop quantum gravity. (shrink)

In an earlier paper I argued that time travellers cannot change the past: alleged models of changing the past either fall into contradiction or else involve avoiding, not changing, the past. Goddu has responded to my argument, maintaining that his hypertime model involves time travellers changing (not avoiding) the past. In the present paper I first discuss what would be required to substantiate the claim that a given model involves changing rather than avoiding the past. I then consider Goddu's hypertime (...) model and an earlier model due to Meiland. I argue that neither author does what would be required to substantiate the claim that the model involves changing (not avoiding) the past. I go on to give reasons for the stronger claim that no-one can present a coherent model and also substantiate the claim that it involves changing (not avoiding) the past. (shrink)

In his paper, The Paradoxes of Time Travel, David Lewis posits a defense for the possibility of time travel by arguing that the grandfather 'paradox' is not, in fact, paradoxical at all. Two alternative solutions to the grandfather paradox are discussed in this paper. The first is a result of Paul Horwich’s reply to Lewis and aims to pit the Lewisian conception of compatibility against Horwich’s improbability defense. Proposed by Nicholas Smith and C.G. Goddu, this theory explains that any attempt (...) at backward time travel will lead to the creation of long strings of improbable coincidences. An alternative thesis of the multiverse is also discussed, wherein it was proposed that instead of traveling into his past, the time traveler enters an alternate, yet completely identical universe. The multiverse thesis did not stand up to any philosophical critique, and it was posited that the thesis changes the nature of the question entirely. It is evident that Lewis’ discussion of the grandfather paradox raises several fundamentally interesting philosophical questions regarding the logical and causal irregularities of changing the past. This paper aims to adress some of these questions through a metaphysical analysis of Lewis' view, backwards causality, and the nature of time itself. (shrink)