I argue that, contrary to an idea to be found in popularizations of time travel, one cannot more easily multiply oneself by taking younger versions of oneself back in time than by travelling back in time on one’s own. The reason is that the suggested multiplication of the traveller is, from a global perspective, only apparent.

_ Paradoxes of Time Travel _ By WassermanRyanOxford University Press, 2018. x + 250 pp.

In 1987, Roy Sorensen coined the term “parahistory” to denote the hypothetical study of evidence retrieved via time travel. Parahistory would thus stand to history rather as parapsychology does to psychology; studying data (in this case artefacts) that are obtained in ways unrecognised by orthodox science. This paper considers future-derived parahistorical artefacts. Past/future asymmetries threaten irresolvable problems in calibrating future objects' periods, in dating future artefacts and insulating them from causal loops. In turn, causal loop objects at best cannot be (...) non-arbitrarily dated at all and at worst, cannot even be artefacts. Even if there can be evidence for travel from the future, any such evidence that appeals to future artefacts faces pervasive difficulties and is very unlikely to be compelling. (shrink)

In 1987, Roy Sorensen coined the term “parahistory” to denote the hypothetical study of evidence retrieved via time travel. Parahistory would thus stand to history rather as parapsychology does to psychology; studying data (in this case artefacts) that are obtained in ways unrecognised by orthodox science. This paper considers future-derived parahistorical artefacts. Past/future asymmetries threaten irresolvable problems in calibrating future objects' periods, in dating future artefacts and insulating them from causal loops. In turn, causal loop objects at best cannot be (...) non-arbitrarily dated at all and at worst, cannot even be artefacts. Even if there can be evidence for travel from the future, any such evidence that appeals to future artefacts faces pervasive difficulties and is very unlikely to be compelling. (shrink)

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.

It has sometimes been suggested that backwards time travel always incurs causal loops. I show that this is mistaken, by describing worlds where backwards time travel occurs and yet no causal loops occur. Arguments that backwards time travel can occur without causal loops have been given before in the literature, but I show that those arguments are unconvincing.

This article argues that the causal loops that occur in some time-travel scenarios and in certain solutions of the theory of relativity are no more mysterious than the infinitely descending causal chains familiar from Newtonian mechanics.

Jan Heylen and Leon Horsten object to my proposed analysis of ordinary-language conditionals by appealing to certain putative counter-examples. In this reply, I explain how, by ignoring my reading of the indicative/subjunctive distinction, their objection misses its target. I also criticize their underlying methodology.

_ Belief: A Pragmatic Picture _ By ZimmermanAaron Z.Oxford University Press, 2018. viii + 180 pp.

Both Lowe and Tsai have presented their own versions of the theory that both indicative and subjunctive conditionals are strict conditionals. We critically discuss both versions and we find each version wanting.

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)

The claim that we are subjects of experience, i.e. beings whose nature is intimately bound up with consciousness, is in many ways a plausible one. There is, however, more than one way of developing a metaphysical account of the nature of subjects. The view that subjects are essentially conscious has the unfortunate consequence that subjects cannot survive periods of unconsciousness. A more appealing alternative is to hold that subjects are beings with the capacity to be conscious, a capacity which need (...) not always be exercised. But this view can itself be developed in more than one way. The option I defend here is that subjects are nothing more than capacities for consciousness, a view I call the. Although the C-theory supplies us with a potentially appealing account of the nature of subjects, there are challenges to be overcome. Olson has argued that identifying ourselves with what are, in effect, parts of human organisms leads to a variety of intolerable problems. I suggest that these problems are by no means insuperable. Bayne and Johnston have argued that identifying subjects with experience-producing systems is confronted with a different difficulty. What if these systems can produce multiple streams of consciousness at once. Whatever else they may be, aren't subjects the kind of thing that can have just one stream of consciousness at a time? In response I argue that this is true in one sense, but not in another. Once this is appreciated, the notion that a subject could have several streams of consciousness at once no longer seems absurd, or impossible. (shrink)

The possibility of continuous backwards time travel—time travel for which the traveler follows a continuous path through space between departure and arrival—gives rise to the double-occupancy problem. The trouble is that the time traveler seems bound to have to travel through his or her younger self as the trip begins. Dowe and Le Poidevin agree that this problem is solved by putting the traveler in motion for a gradual trip to the past. Le Poidevin goes on to argue, however, that (...) the gradual trip gives rise to the Cheshire cat problem, a concern about whether the traveler survives the gradual trip. We address the Cheshire cat problem by proposing and considering new continuity constraints on identity over time. Along the way, we come upon an endurantist conception of temporal parts. (shrink)