Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn't (...) this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics. (shrink)

I defend what may loosely be called an eliminativist account of causation by showing how several of the main features of causation, namely asymmetry, transitivity, and necessitation, arise from the combination of fundamental dynamical laws and a special constraint on the macroscopic structure of matter in the past. At the microscopic level, the causal features of necessitation and transitivity are grounded, but not the asymmetry. At the coarse-grained level of the macroscopic physics, the causal asymmetry is grounded, but not the (...) necessitation or transitivity. Thus, at no single level of description does the physics justify the conditions that are taken to be constitutive of causation. Nevertheless, if we mix our reasoning about the microscopic and macroscopic descriptions, the structure provided by the dynamics and special initial conditions can justify the folk concept of causation to a significant extent. I explain why our causal concept works so well even though at bottom it is comprised of a patchwork of principles that don't mesh well. (shrink)

David Lewis famously defends a counterfactual theory of causation and a non-causal, similarity-based theory of counterfactuals. Lewis also famously defends the possibility of backward causation. I argue that this combination of views is untenable—given the possibility of backward causation, one ought to reject Lewis's theories of causation and counterfactuals.

This paper argues that the most famous objection to backward time travel can carry no weight. In its classic form, the objection is that backward time travel entails the occurrence of impossible things, such as auto-infanticide—and hence is itself impossible. David Lewis has rebutted the classic version of the objection: auto-infanticide is prevented by coincidences, such as time travellers slipping on banana peels as they attempt to murder their younger selves. I focus on Paul Horwich‘s more recent version of the (...) objection, according to which backward time travel entails not impossible things, but improbable ones—such as the string of slips on banana peels that would be required to stop a determined auto-infanticidal maniac from murdering her younger self—and hence is itself highly improbable. I argue that backward time travel does not entail unusual numbers of coincidences; and that, even if it did, that would not render its occurrence unlikely. (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)

David Lewis’s semantics for counterfactuals remains the standard view. Yet counter-examples have emerged, which suggest a need to invoke causal independence, and thus threaten conceptual circularity. I will review some of these counter-examples (§§1–2), illustrate how causal independence proves useful (§3), and suggest that any resulting circularity is unproblematic (§4).

Among the most important questions in the metaphysics of science are "What are the natures of fundamental laws and chances?" and "What grounds the direction of time?" My aim in this paper is to examine some connections between these questions, discuss two approaches to answering them and argue in favor of one. Along the way I will raise and comment on a number of issues concerning the relationship between physics and metaphysics and consequences for the subject matter and methodology of (...) metaphysics. (shrink)

I insist that I was able to raise my hand, and I acknowledge that a law would have been broken had I done so, but I deny that I am therefore able to break a law. To uphold my instance of soft determinism, I need not claim any incredible powers. To uphold the compatibilism that I actually believe, I need not claim that such powers are even possible. My incompatibilist opponent is a creature of fiction, but he has his prototypes (...) in real life. He is modeled partly after Peter van Inwagen and partly on myself when I first worried about van Inwagen's argument against compatibilism. (shrink)

The most potentially powerful objection to the possibility oftime travel stems from the fact that it can, under the right conditions, give rise to closedcausal loops, and closed causal loops can be turned into self-defeating causal chains;folks killing their infant selves, setting out to destroy the world before they were born,and the like. It used to be thought that such chains present paradoxes; the receivedwisdom nowadays is that they give rise to physical anomalies in the form of inexplicably correlated events. (...) I argue against the received wisdom. I can find nothing in them that argues against the possibility (even, the probability) of time travel. (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)

Fundamental physics makes no clear use of causal notions; it uses laws that operate in relevant respects in both temporal directions and that relate whole systems across times. But by relating causation to evidence, we can explain how causation fits in to a physical picture of the world and explain its temporal asymmetry. This paper takes up a deliberative approach to causation, according to which causal relations correspond to the evidential relations we need when we decide on one thing in (...) order to achieve another. Tamsin's taking her umbrella is a cause of her staying dry, for example, if and only if her deciding to take her umbrella for the sake of staying dry is adequate grounds for believing she'll stay dry. This correspondence explains why causation matters: knowledge of causal structure helps us make decisions that are evidence of outcomes we seek. The account also explains why we can control the future and not the past, and why causes come before their effects. When agents properly deliberate, their decisions can never count as evidence for any outcomes they may seek in the past. From this it follows that causal relations don't run backwards. This deliberative asymmetry is itself traced back to asymmetries of evidence and entropy, providing a new way of deriving causal asymmetry from temporally symmetric laws. (shrink)

One of the most striking features of causation is that causes typically precede their effects – the causal arrow is strongly aligned with the temporal arrow. Why should this be so? We offer an opinionated guide to this problem, and to the solutions currently on offer. We conclude that the most promising strategy is to begin with the de facto asymmetry of human deliberation, characterised in epistemic terms, and to build out from there. More than any rival, this subjectivist approach (...) promises to demystify the asymmetry, temporal orientation, and deliberative relevance of causal judgements. (shrink)