This is the text of The Lindley Lecture for 1962, given by Arthur N. Prior (1914-1969).

Different versions of the A-theory of time are traditionally defined in terms of whether everything is present, or whether there are also past and future things. In this paper I argue that the traditional way of defining A-theories should be abandoned. I focus on the traditional definition of presentism, according to which always, everything is present. First, I argue that there are good reasons to reject all the most plausible interpretations of the predicate ‘is present’ as it appears in the (...) traditional definition of presentism. It follows that there are also good reasons to reject the most plausible interpretations of the traditional definitions of the other A-theories. I then argue that there is a better way of defining the A-theories, in terms of the question of whether existence has a beginning and an end. Finally, I argue that what goes for the traditional definition of presentism goes for the traditional definition of its modal analogue actualism, according to which necessarily, everything is actual : there are good reasons to reject the traditional definition of actualism in favour of a definition in terms of contingent existence. (shrink)

We argue that presentism, understood as a view about time and existence, can perspicuously be defined in opposition to all other familiar contenders without appeal to any notion of presentness or cognate notions such as concreteness. Given recent worries about the suitability of such notions to cut much metaphysical ice, this should be welcomed by presentism's defenders. We also show that, irrespective of its sparse ideology, the proposed formulation forestalls any deviant interpretation at odds with the view it aims to (...) capture. (shrink)

Ted Sider aptly and concisely states the self-visitation paradox thus: 'Suppose I travel back in time and stand in a room with my sitting 10-year-old self. I seem to be both sitting and standing, but how can that be?' (2001, 101). I will explore a relativist resolution of this paradox offered by, or on behalf of, endurantists.1 It maintains that the sitting and the standing are relative to the personal time or proper time of the time traveler and is intended (...) to yield the result that Ted is sitting at a certain initial personal/proper time but is not standing relative to that time. Similarly, it is also supposed to yield that Ted is standing relative to a later personal/proper time, but not sitting relative to that .. (shrink)

Four-dimensionalism is the thesis that everyday objects, such as you and me, are space-time worms that persist through time by having temporal parts none of which is identical to the object itself. Objects are aggregates or sums of such temporal parts. The main virtue of fourdimensionalism is that it solves—or does away with—the problem of identity through change.1 The main charge raised against it is that it is inconsistent with the thesis according to which there is change in the world.2 (...) If this charge could be sustained, then we would need compelling arguments in support of the view that things are four-dimensional, since the view that there is no change in the world conflicts with so many of our most well-supported common-sense beliefs. In what follows, however, I want to show that, contrary to what is usually believed, fourdimensionalism does not entail a changeless world. (shrink)

In this paper, I examine various theories of persistence through time under presentism. In Part I, I argue that both perdurantist views (namely, the worm view and the stage view) suffer, in combination with presentism, from serious difficulties and should be rejected. In Part II, I discuss the presentist endurantist view, to see that it does avoid the difficulties of the perdurantist views, and consequently that it does work, but at a price that some may consider as being very high: (...) its ontological commitments to platonic universals and to the substratum theory, that as we shall see follow from the combination of endurantism with presentism, will perhaps not be to everyone's taste. (shrink)

This book is a defense of modal realism; the thesis that our world is but one of a plurality of worlds, and that the individuals that inhabit our world are only a few out of all the inhabitants of all the worlds. Lewis argues that the philosophical utility of modal realism is a good reason for believing that it is true.

Ryan Wasserman explores a range of fascinating puzzles raised by the possibility of time travel, with entertaining examples from physics, science fiction, and popular culture, and he draws out their implications for our understanding of time, tense, freedom, fatalism, causation, counterfactuals, laws of nature, persistence, change, and mereology.

Ulrich Meyer defends a novel theory about the nature of time, and argues against the consensus view that time and space are fundamentally alike. He presents the first comprehensive defense of a 'modal' account, which emphasizes the similarities between times and possible worlds in modal logic, and is easily reconciled with the theory of relativity.

Kadri Vihvelin, in "What time travelers cannot do" (Philos Stud 81: 315-330, 1996), argued that "no time traveler can kill the baby who in fact is her younger self, because (V1) "if someone would fail to do something, no matter how hard or how many times she tried, then she cannot do it", and (V2) if a time traveler tried to kill her baby self, she would always fail. Theodore Sider (Philos Stud 110: 115-138, 2002) criticized Vihvelin's argument, and Ira (...) Kiourti (Philos Stud 139: 343-352, 2008) criticized both Vihvelin's argument and Sider's critique. I present a critique of Vihvelin's argument different from both Sider's and Kiourti's critiques: I argue in a novel way that both V1 and V2 are false. Since Vihvelin's argument might be understood as providing a challenge to the possibility of time travel, if my critique succeeds then time travel survives such a challenge unscathed. (shrink)

Here I examine some recent attempts to provide a new way of thinking about the philosophy of time that question the central role of ‘presentness’ within the definition of presentism. The central concern raised by these critics turns on the intelligibility and theoretical usefulness of the term ‘is present’. My overarching aim is to at least challenge such concerns. I begin with arguments due to Deasy. Deasy develops a view that he calls ‘transientism’ and that he takes to be a (...) well-motivated version of presentism. I show that both this way of thinking about presentism and the argument supposedly motivating it all fail. I then move to an argument due to Correia and Rosenkrantz. Correia and Rosenkrantz purport to show that presentism can be salvaged without making recourse to the term ‘is present’. I demonstrate that their arguments fail. I then move on to a view, proposed and defended by Merricks, Tallant, and Zimmerman, and show that it has the wherewithal to meet the challenges raised by Williamson who, as noted above, raises genuine concerns about our capacity to define presentism. (shrink)

In this paper I argue in favour of a new definition of presentism that I call ‘existence presentism’ (EP). Typically, presentism is defined as the thesis that ‘only present objects exist’, or ‘nothing exists that is non-present’.1 I assume these statements to be equivalent. I call these statements of presentism ‘conventional presentism’ (CP). First, in §2, I rehearse arguments due to Ulrich Meyer that purport to show that presentism is not adequately defined as CP. In §§2.1–2.4 I show that considerations (...) of the sort raised by Meyer infect attempts to define presentism, due to Thomas Crisp, Tom Stoneham, and Theodore Sider. Thus the first half of the paper is constituted by a negative project that looks to show how extant definitions of presentism fail. In the second half of the paper, I continue in the negative theme in §3, before laying out EP and demonstrating that it solves the problems faced by CP and offering explanations of how to make sense of certain key notions that lie at the heart of EP. I conclude that EP is preferable to CP. (shrink)

According to the Weak Supplementation Principle (WSP)—a widely received principle of mereology—an object with a proper part, p , has another distinct proper part that doesn't overlap p . In a recent article in this journal, Nikk Effingham and Jon Robson employ WSP in an objection to endurantism. I defend endurantism in a way that bears on mereology in general. First, I argue that denying WSP can be motivated apart from the truth of endurantism. I then go on to offer (...) an explanation of WSP's initial appeal, argue that denying WSP fails to have untoward consequences for the rest of mereology, and show that the falsity of WSP is consistent with a primary guiding thought behind it. (shrink)

Some say that presentism precludes time travel into the past since it implies that the past does not exist, but this is a bad argument. Presentism says that only currently existing entities exist, and that the only properties and relations those entities instantiate are those that they currently instantiate. This does in a sense imply that the past does not exist. But if that precluded time travel into the past, it would also preclude the one-second-per-second “time travel” into the future (...) that is ordinary persistence, for presentism accords the future the same ontological status as the past. Instead of quantifying over past and future objects and events, presentists speak a tensed language, regimented with primitive sentential tense operators. For a presentist, a persisting person is one who did exist, and who will exist. Regimented, these claims become: it was the case that she exists, and it will be the case that she exists. The presentist may then apply the same strategy to time travel proper. Suppose Katy travels back to the time of the dinosaurs. The presentist can say that it was the case two hundred million years ago that Katy exists. This claim, which consists of a present-tense statement “Katy exists” embedded within the past tense operator it was the case two hundred million years ago that, is exactly the sort of statement about time that a presentist is free to accept. This has all been made clear by Simon Keller and Michael Nelson ( ). In addition to rebutting the bad argument against the consistency of presentism and time travel, Keller and Nelson argue positively in favor of consistency by showing how to translate David Lewis’s ( ) account of time travel into the presentist’s tensed language. The appearance of con ict between presentism and time travel, they argue, is due only to the fact that most defenders of time travel (for example Lewis) have tended to phrase their defenses in nonpresentist terms. As much as I applaud their rebuttal of the bad argument, I wish to sound a note of caution.. (shrink)

Persistence through time is like extension through space. A road has spatial parts in the subregions of the region of space it occupies; likewise, an object that exists in time has temporal parts in the various subregions of the total region of time it occupies. This view — known variously as four dimensionalism, the doctrine of temporal parts, and the theory that objects “perdure” — is opposed to “three dimensionalism”, the doctrine that things “endure”, or are “wholly present”.1 I will (...) attempt to resolve this dispute in favor of four dimensionalism by means of a novel argument based on considerations of vagueness. But before argument in this area can be productive, I believe we must become much clearer than is customary about exactly what the dispute is, for the usual ways of formulating the dispute are flawed, especially where three dimensionalism is concerned. (shrink)

Presentism is usually understood as the thesis that only the present exists whereas the rival theory of eternalism is usually understood as the thesis that past, present, and future things are all equally real. The significance of this debate has been threatened by the so-called triviality objection, which allegedly shows that the presentist thesis is either trivially true or obviously false: Presentism is trivially true if it is read as saying that everything that exists now is present, and it is (...) obviously false if read as saying that everything that has existed, exits or will exist is present. If eternalism is taken as the negation of presentism, it is also either trivially false or obviously true. In this paper, I try to respond to the triviality objection on behalf of presentism. In second section, I will examine how the argument proceeds. In third section, I will reflect on three possible ways to respond but will argue that none of them succeeds in giving a satisfactory solution. I will then try to clarify the core idea of presentism and to suggest that if we characterise presentism accurately, the problem will disappear. In fourth section, I will offer a plausible definition of presentism and will show how it can avoid the triviality objection and demonstrate why it is advantageous to accept the version of presentism I offer. (shrink)

It is sometimes argued that there is an analogy between time and modality: What is true of time, mutatis mutandis, should be true of modality, and vice versa. However, I think that the importance of this analogy has not been truly appreciated in the literature. In this paper, I try to offer a plausible account of the relationship between time and modality based on what is known as presentist ersatzism. If the attempt succeeds, it will be shown that ersatzists about (...) time are better able to explain what possible worlds are. (shrink)

Typical presentism asserts that whatever exists is present. Moderate presentism more modestly claims that all events are present and thus acknowledges past and future times understood in a substantivalist sense, and past objects understood, following Williamson, as “ex-concrete.” It is argued that moderate presentism retains the most valuable features of typical presentism, while having considerable advantages in dealing with its most prominent difficulties.

It is often thought that presentism is incompatible with time travel. I will argue that this common view is incorrect. Specifically, I will argue that presentism is compatible with some stories that involve closed timelike curves, and that some of these stories are time-travel stories.

It is possible to wholly exist at multiple spatial locations at the same time. At least, if time travel is possible and objects endure, then such must be the case. To accommodate this possibility requires the introduction of a spatial analog of either relativising properties to times—relativising properties to spatial locations—or of relativising the manner of instantiation to times—relativising the manner of instantiation to spatial locations. It has been suggested, however, that introducing irreducibly spatially relativised or spatially adverbialised properties presents (...) some difficulties for the endurantist. I will consider an objection according to which embracing such spatially relativised properties could lead us to reject mereology altogether in favour of a metaphysics according to which objects are wholly present at every space-time point at which they exist. I argue that although such a view is coherent, there are some good reasons to reject it. Moreover, I argue that the endurantist can introduce spatially relativised or adverbialised properties without conceding that objects lack spatial parts. Such a strategy has the additional advantage that it allows the endurantist not only to explain time travel, but also to reconcile our competing intuitions about cases of fission. (shrink)

It is possible to wholly exist at multiple spatial locations at the same time. At least, if time travel is possible and objects endure, then such must be the case. To accommodate this possibility requires the introduction of a spatial analog of either relativising properties to times—relativising properties to spatial locations—or of relativising the manner of instantiation to times—relativising the manner of instantiation to spatial locations. It has been suggested, however, that introducing irreducibly spatially relativised or spatially adverbialised properties presents (...) some difficulties for the endurantist. I will consider an objection according to which embracing such spatially relativised properties could lead us to reject mereology altogether in favour of a metaphysics according to which objects are wholly present at every space-time point at which they exist. I argue that although such a view is coherent, there are some good reasons to reject it. Moreover, I argue that the endurantist can introduce spatially relativised or adverbialised properties without conceding that objects lack spatial parts. Such a strategy has the additional advantage that it allows the endurantist not only to explain time travel, but also to reconcile our competing intuitions about cases of fission. (shrink)

Steven Hales constructs a novel argument against the possibility of presentist time travel called the suicide machine argument. Hales argues that if presentism were true, then time travel would result in the annihilation of the time traveler. But such a consequence is not time travel, therefore presentism cannot allow for the possibility of time travel. This paper argues that in order for the suicide machine argument to succeed, it must make (at least) one of two assumptions, each of which beg (...) the question. The argument must either assume that the sequence of moments is invariant, or that time travel requires distinct, co-instantiated moments. Because the former disjunct assumes that presentist time travel is impossible and the latter assumes that presentism is impossible, the suicide machine argument fails. (shrink)

I assess the debate over the Suicide Machine Argument. There are several lessons to be learned from this debate. First, there is a fruitful distinction to be made,between tensed and tenseless versions of presentism, despite the temptation to suppose that presentism is a tensed theory of time. Second, once we’ve made the distinction between different kinds of presentism, it is clear that Licon’s objection protects the tenseless version of presentism from the Suicide Machine Argument; however, the argument is still effective (...) against the tensed version. Finally, I argue that if the presentist wants to remain a card carrying presentist, in the face of the challenge posed by Hales, then she must abandon her commitment to tense. (shrink)

A solution to the problem of intrinsic change for enduring things should meet three conditions. It should not replace monadic intrinsic properties by relations. It should not replace the having simpliciter of properties by standing in some relation to them. It should not rely on an unexplained notion of having an intrinsic property at a time. Johnston's solution satisfies the first condition at the expense of the second. Haslanger's solution satisfies the first and second at the expense of the third.

In her (1996) Kadri Vihvelin argues that autoinfanticide is nomologically impossible and so that there is no sense in which time travelers are able to commit it. In response, Theodore Sider (2002) defends the original Lewisian verdict (Lewis 1976) whereby, on a common understanding of ability, time travelers are able to kill their earlier selves and their failure to do so is merely coincidental. This paper constitutes a critical note on arguments put forward by both Sider and Vihvelin. I argue (...) that although Sider’s criticism starts out promisingly he doesn’t succeed in establishing that Vihvelin’s analysis fails, because (a) he neglects to rule out a class of counterfactuals to which Vihvelin’s sample-case may belong; and (b) (together with Lewis) he is wrong to suggest that future facts are irrelevant in the evaluation of time travelers’ abilities. I show instead that Vihvelin’s argument is viciously circular, indicating that even if there are nomological constraints on autoinfanticide these cannot be established a priori. (shrink)

In this paper I offer a rejoinder to the criticisms raised by Jimmy Alfonso Licon in “No Suicide for Presentists: A Response to Hales.” I argue that Licon's concerns are misplaced, and that his hypothetical presentist time machine neither travels in time nor saves the life of the putative traveler. I conclude that sensible time travel is still forbidden to presentists.

In the present paper, I offer a new argument to show that presentism about time is incompatible with time travel. Time travel requires leaving the present, which, under presentism, contains all of reality. Therefore to leave the present moment is to leave reality entirely; i.e. to go out of existence. Presentist “time travel” is therefore best seen as a form of suicide, not as a mode of transportation. Eternalists about time do not face the same difficulty, and time travel is (...) compossible with eternalism. (shrink)

In this paper I defend the compossibility of presentism and time travel from two objections. One objection is that the presentist’s model of time leaves nowhere to travel to; the second objection attempts to equate presentist time travel with suicide. After targeting some misplaced scrutiny of the first objection, I show that presentists have the resources to account for the facts that make for time travel on the traditional Lewisian view. In light of this ability, I argue that both of (...) the objections fail. (shrink)

According to the prevailing sentiment, changing the past is logically impossible. The prevailing sentiment is wrong. In this paper, I argue that the claim that changing the past entails a contradiction ultimately rests upon an empirical assumption, and so the conclusion that changing the past is logically impossible is to be resisted. I then present and discuss a model of time which drops the empirical assumption and coherently models changing the past. Finally, I defend the model, and changing the past, (...) against objections. (shrink)

According to the prevailing sentiment, changing the past is logically impossible. The prevailing sentiment is wrong. In this paper, I argue that the claim that changing the past entails a contradiction ultimately rests upon an empirical assumption, and so the conclusion that changing the past is logically impossible is to be resisted. I then present and discuss a model of time which drops the empirical assumption and coherently models changing the past. Finally, I defend the model, and changing the past, (...) against objections. (shrink)

Some philosophers argue that any attempt to model changing the past will either be contradictory or really model avoiding the past. Using Nicholas Smith's (1997) argument as a basis, I formulate a generic version of this Avoidance Argument. I argue that the Avoidance Argument fails because (i) it involves an equivocation of what is meant by ‘bifurcation of the time of an event’ and (ii) resolving the equivocation results in the falsity of at least one of the premises. Hence, the (...) Avoidance Argument will not support the claim that changing the past is logically impossible. (shrink)

In this paper, I defend my original objection to Hales’ suicide machine argument against Hales’ response. I argue Hales’ criticisms are either misplaced or underestimate the strength of my objection; if the constraints of the original objection are respected, my original objection blocks Hales’ reply. To be thorough, I restate an improved version of the objection to the suicide machine argument. I conclude that Hales fails to motivate a reasonable worry as to the supposed suicidal nature of presentist time travel.

This paper argues that, in light of certain scenarios involving time travel, Sider’s definition of ‘instantaneous temporal part’ cannot be accepted in conjunction with a semantic thesis that perdurantists often assume. I examine a rejoinder from Sider, as well as Thomson’s alternative definition of ‘instantaneous temporal part’, and show how neither helps. Given this, we should give up on the perdurantist semantic thesis. I end by recommending that, once we no longer accept such semantics, we should accept a new set (...) of definitions, which are superior in certain respects to Sider’s original set. (shrink)

In this paper, we argue that time travel is problematic for the endurantist. For it appears to be possible, given time travel, to construct a wall out of a single time travelling brick. This commits the endurantist to one of the following: (a) the wall is composed of the time travelling brick many times over; (b) the wall does not in fact exist at all; (c) the wall is identical to the brick. We argue that each of these options is (...) unsatisfactory. (shrink)

I have previously argued in a paper with Robson that a particular time travel scenario favours perdurantism over endurantism on the grounds that endurantists must give up on the Weak Supplementation Principle. Smith has responded, arguing that the reasons we provided are insufficient to warrant this conclusion. This paper agrees with that conclusion (for slightly different reasons: that even the perdurantist has to give up on the Weak Supplementation Principle) but argues that the old argument can be supplanted with a (...) new one. (shrink)

Truth and Ontology concludes that some truths do not depend on being in any substantive way at all.

Space and time are the most fundamental features of our experience of the world, and yet they are also the most perplexing. Does time really flow, or is that simply an illusion? Did time have a beginning? What does it mean to say that time has a direction? Does space have boundaries, or is it infinite? Is change really possible? Could space and time exist in the absence of any objects or events? What, in the end, are space and time? (...) Do they really exist, or are they simply the constructions of our minds? Robin Le Poidevin provides a clear, witty, and stimulating introduction to these deep questions and many other mind-boggling puzzles and paradoxes. He gives a vivid sense of the difficulties raised by our ordinary ideas about space and time, but he also gives us the basis to think about these problems independently, avoiding large amounts of jargon and technicality. His book is an invitation to think philosophically rather than a sustained argument for particular conclusions, but Le Poidevin does advance and defend a number of controversial views. He argues, for example, that time does not actually flow, that it is possible for space and time to be both finite and yet be without boundaries, and that causation is the key to an understanding of one of the deepest mysteries of time: its direction. Drawing on a variety of vivid examples from science, history, and literature, Travels in Four Dimensions brings to life some of the most profound questions imaginable. (shrink)