The Priveleged Present

The Privileged Present: Defending an ‘A-theory’ of Time Dean Zimmerman Draft: August 15, 2006 Forthcoming in Contemporary Debates in Metaphysics, ed. by Theodore Sider, John Hawthorne, and Dean Zimmerman (Malden, Mass.: Blackwell) Introduction The following questions go to the heart of the deepest metaphysical disagreement about the nature of time: (i) Are there objective differences between what is past, present, and future? (ii) Are present events and things somehow more “real” than those wholly in the past or future? I should like to respond, “Yes”, to both questions. Affirmative answers sound obvious and commonsensical, at least to me. Indeed, I suspect that, for many of us, belief in a deep distinction between past, present, and future can be given up briefly, if at all; and then only by a mighty effort of will! Over the course of the 20th Century, however, more and more philosophers have argued for negative answers to these questions. In many quarters, the impulse to posit a deep difference between past, present, and future is now taken to be no better grounded than the impulse to suppose that there is an objective “downward” direction, the same everywhere in the universe; or that the earth is stationary, while the sun, moon, and stars are not. There are three parts to my defense of affirmative answers to (i) and (ii). First, I describe a number of theories of time that answer “Yes” to (i), and raise a worry about the ones that do not also answer “Yes” to (ii). Then I rebut a kind of skeptical criticism of the metaphysics of time I have been describing; the skeptic alleges that the metaphysicians who are arguing about whether to answer “Yes” or “No” to these

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questions are just talking past one another. Finally, I assess reasons to reject or accept a metaphysics of time that answers (i) and (ii) affirmatively. I consider some metaphysical and scientific arguments against affirmative answers, and find them wanting. More positively, I argue that belief in a real difference between past, present, and future has a certain positive status: it is “innocent until proven guilty”, and guilt remains unproven.

A-theories and B-theories At the beginning of the 20th century, J. McT. E. McTaggart introduced some (arbitrarily chosen and rather bland) terminology that has been used ever since. He gave the name “A-series” to “that series of positions which runs from the far past through the near past to the present, and then from the present through the near future to the far future, or conversely”; and the name “B-series” to “[t]he series of positions which runs from earlier to later, or conversely”.1 McTaggart’s labels have stuck, and been put to further use. The properties being past, being present, and being future, are generally called the “Aproperties”. The relations of being earlier than, being later than, and being simultaneous with, are the “B-relations”. Metaphysicians who argue about time are divided by their attitude toward the Aproperties and B-relations. Some regard A-properties as fundamental, and B-relations as derivative — they are “A-theorists” — while others regard the B-relations as fundamental, and the A-properties as derivative — they are the “B-theorists”. To be an A-theorist is to believe in some sort of objective distinction between what is present and what is past and what is future. A-theorists answer question (i), above, in the affirmative: 1 McTaggart, 1998, p. 68. The terms “A series” and “B series” were introduced in McTaggart, 1908.

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the present is distinguished from past and future in a way that is not relative to any other temporal thing, such as a conversation, a time, or a frame of reference. To be a Btheorist, on the other hand, is to deny the objectivity of our talk about past, present, and future. When we say that certain events and times are past, present, or future, we are not describing the world “as it is in itself”; we are merely describing the temporal locations of things relative to one particular (but not at all special) temporal location — the point in time at which the description is being given. A-theorists disagree about the exact nature of the objective differences they posit between past, present, and future. But most of them would give an affirmative answer to (ii), as well: The difference between present events and things, as opposed to those in the past or future, is that the present events and things are in some sense “more real”. The future is “open”, a realm of “mere possibilities”. The past is a realm of things that have “faded away”.

Competing Versions of the A-theory Consider some event that is happening, right now — for example, your reading the words in this very sentence. Too late! That event is over; it is already past. Consider some individual that exists now but soon will not — for example, a positron within the sun. Too late again! It collided with an electron, and both were annihilated in a violent explosion that left behind only a neutrino. The neutrino, with its tiny mass, could hardly be composed of the electron and positron that went into the reaction. So our poor positron is no more; it has utterly ceased to be. I expect that most of my readers would happily agree to the following claim about the two short-lived things I described:

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Because they are entirely in the past, both the event that was your reading of the sentence, and the positron, do not exist. Another sensible-sounding claim: the event of your reading the final sentence in this paper does not exist; nor do the positrons that will be created by proton fusion within the sun later today. And that is just what it is for an event or thing to “move” from the future into the present, and from the present into the past: It is to come into existence and then go out of existence. “Presentism” is the usual name given to a version of the A-theory that accepts, at face value, this account of the differences among past, present, and future things. Suppose that a moment of time (say, the instant at which you finish reading this parenthetical clause) is just the sum of all the events going on right then — a popular view, and surely one thing that could be meant by the phrase “moment of time”. In that case, past and future times would have to be composed of events that, according to the presentist, don’t exist. Saint Augustine drew the logical presentist conclusion: “it is abundantly clear that neither the future nor the past exist, and therefore it is not strictly correct to say that there are three times, past, present, and future.”2 Presentists, then, maintain that there exist only present events, individuals, and times — assuming times are sums or collections of simultaneous events. Some A-theorists are not presentists. Augustine imagines the following sort of objection to presentism: We can, after all, foresee that certain things will happen or come into existence, and we can remember things that did happen or did exist. And so past and future things must have some kind of reality, in order for us to stand in these relations to them. “Otherwise, how do prophets see the future, if there is not yet a future to be seen?

2 Augustine, p. 269 (Book XI, §20).

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It is impossible to see what does not exist. In the same way people who describe the past could not describe it correctly unless they saw it in their minds, and if the past did not exist it would be impossible for them to see it at all.” Accepting this conclusion would lead to a very different sort of A-theory: the view “that past and future do exist, but that time emerges from some secret refuge when it passes from the future to the present, and goes back into hiding when it moves from the present to the past.”3 If an A-theorist follows this line of reasoning, she must admit that past and future things, events, and times all exist. Reality consists of the events of many times, in something like a four-dimensional array. But the A-theory implies that only some events are genuinely present. So the present becomes a kind of “moving spotlight” playing over a four-dimensional universe. The image of the spotlight was introduced by C. D. Broad in 1923, in the form of a “policeman’s bull’s-eye” — a lamp that focuses a beam of light:

We are naturally tempted to regard the history of the world as existing eternally in a certain order of events. Along this, and in a fixed direction, we imagine the characteristic of presentness as moving, somewhat like the spot of light from a policeman’s bull’s-eye traversing the fronts of the houses in a street. What is illuminated is the present, what has been illuminated is the past, and what has not yet been illuminated is the future.4

3 Augustine, p. 267 (Book XI, §18).

4 Broad, p. 59.

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On the moving-spotlight A-theory (which, incidentally, Broad did not endorse), future and past events and things are just as much a part of “the furniture of the world” as present events and things. But there is something special about the ones that are present — they are “lit up” in some way. Before and after being illuminated, they reside in darkness — in a “secret refuge”, to return to Augustine’s metaphor. An intermediate form of the A-theory accepts the existence of past and present events, things, and times; but denies the reality of the future. This is the “growing block” view of time defended by C. D. Broad: Although both past and present things and events exist, present events differ from past ones in that there are (ever so briefly!) no events later than them. “[T]he essence of a present event is, not that it precedes future events, but that there is quite literally nothing to which it has the relation of precedence.” 5 What makes present things special, according to the moving-spotlight or growingblock A-theories? How do things change as they cease to be present? In his arguments against the reality of time, McTaggart took a moving-spotlight theory as his primary target; and he describes the moving spotlight of “presentness” as a property that events gain or lose without undergoing any other significant changes; and a moving-spotlighter might conceive of the passage of presentness in this extremely thin way. Broad’s growing-block A-theory is similar to a thin spotlighter’s view; it implies that things and

5 Broad, p. 66. Broad’s defense of the growing block view is reprinted as Ch. 12 of van Inwagen and

Zimmerman, 2007.

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events do not change significantly when they cease to be present. For Broad, being present is simply a matter of being on the “cutting edge” of a growing universe spread out in four dimensions; ceasing to be present, and becoming past, involves no intrinsic change whatsoever: “Nothing has happened to the present by becoming past except that fresh slices of existence have been added to the total history of the world.”6 These two “thin” conceptions of the nature of the present are A-theories, to be sure; they give an affirmative answer to (i), the first of the two questions with which I began. They do not, however, give an unqualified “Yes” to the second question. The moving-spotlighter, as imagined by McTaggart, answers (ii) negatively: Past and future things are in no meaningful sense “less real” than present things. They lack a peculiar property called “presentness”; but, as they acquire and lose this property, they undergo no interesting changes of any other kind. Unlike McTaggart’s moving-spotlighter, Broad grants that future events and things are less real — for, on the growing block theory, they do not exist at all. But Broad gives a negative answer to part of question (ii): Past things and events are in no sense “less real” than present ones; they undergo no intrinsic changes, but only relational ones, as “fresh slices of existence” are added to the fourdimensional block of which they are parts. Neither of these is a version of the A-theory I should care to defend. When I notice that a headache, or some other painful episode, has become part of the past, I am relieved that this is so; and when a pleasant experience becomes past, I am often disappointed. If a theory of time makes such changes in attitude utterly mysterious, we should have grave doubts about its adequacy. And neither of the two versions of the A6 Broad, p. 66.

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theory under consideration can explain why we care so much about whether things are present or past.7 If a pain is just as intrinsically painful when it has the spotlight upon it, or when it is on the cutting edge, why should the passage of the spotlight or the adding of slices change our attitude toward it? And even if we did care about these rather obscure changes, how could we ever know when they have occurred? If either view is right, the mental and physical states that now characterize me will continue, intrinsically just the same, for the rest of time. If my state of mind does not change intrinsically when it becomes past, then, even if I am now correct in my beliefs about what is presently happening, won’t these beliefs quickly become mostly false, and go right on being mostly false for all eternity?8 The A-theory certainly loses much of its appeal, if it induces such skepticism. Some recent defenders of the moving-spotlight or growing-block have ready answers to the questions: “Why should we care whether things are present?” and “How could we know what time is present?” They maintain that at least some objects and events, when they are present, are intrinsically different from the way they are when not present; some things change, drastically, when they cease to be “lit up” by presentness or when they become embedded within the growing block. Today’s spotlighters and moving-blockers generally agree that when an object ceases to be present, it loses a great many of its most interesting properties. Quentin Smith and Timothy Williamson, for example, believe that the entire block of past, present, and future individuals and events exists; but that, when objects and events pass 7 Arthur Prior is famous for lodging a similar objection against the B-theory of time, under the slogan

“Thank goodness that’s over!” (see Prior, p. 50). 8 The question is asked by Braddon-Mitchell and Merricks.

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from being future to present to past, they change in much more than just their Aproperties — i.e., their presentness, or their degree of pastness or futurity. For example, they think that future things and events are not located anywhere in space until they are present; and, when past, they again become nonspatial — though of course it remains true that they once occupied space. These spotlighters strip past and future objects and events of all their interesting intrinsic properties, too. A table yet-to-be-made has no shape or mass or color; and when it is destroyed, it will lack these properties as well. On Williamson’s view, things even cease to belong to (what one would ordinarily have thought to be) their essential kinds: “A past table is not a table that no longer exists; it is no longer a table.”9 Robert M. Adams and Peter Forrest are growing-blockers with similar views about the way in which objects and events change when they cease to be on the “cutting edge”, and thereby become past. These versions of the A-theory seem to be able to do justice to the feeling that what’s in the past is over and done with, and that what’s in the future only matters because it will eventually be present. They can also plausibly explain how we know what time is present. But if past headaches are to be much better than present ones, these Atheorists must say things like: A headache is only truly painful when it is present; yesterday’s headache, although it exists, is no longer painful. It has a past-oriented property, having been painful — a sort of backwards-looking relation to the property being painful. But actually being painful is a matter of simply having the property itself, not standing in some other relation to it; and that’s why it no longer concerns us. Similar things can be said about someone’s past observation that the present time (i.e., the spotlit 9 Williamson, p. 195.

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time, or the cutting edge) is exactly noon. If this judgment about what time is present continues to exist even though it is past; then either it is almost always false, or else the observation of an event (such as the coincidence of watch hands) by a person only actually occurs when the observation is present. The wise A-theorist should take the latter alternative: once the person’s observation has ceased to be present, it is no longer in any sense occurring. And the strategy can be extended to all the interesting properties of events and objects; to be truly loud, tall, hungry, etc. is to be presently loud, tall, hungry, etc. Although this view makes sense of our relief when pain is past, and of our knowledge of what time is present, it has less appealing consequences as well. Headaches can exist but not be truly painful; a horse can exist although it is not actually alive or even spatially located. What’s left of these past things and events is extremely thin: a physical object can survive a change in which it ceases to have any shape or size, an explosion can continue to exist when all its energy has dissipated. If the spotlighter or growing-blocker tries to tone down her claims about the thinness of past things — if she says, for instance, that past headaches are still “sort of painful” and that we remain “sort of aware” of what we observe even when our observations are no longer present — then it once again becomes difficult to answer the questions, “Why care that the headache is past?” and “How do we know what time is present?” For the presentist, answering these questions is easy. Past headaches do not exist; consequently, they have no properties whatsoever, including being painful. And we can be sure that we are present, and not buried deep in the past of a four-dimensional block; for, as Braddon-Mitchell points out, “according to the presentist all that exists is the

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present, so the fact that we know we exist guarantees that we are in the present.” (p. ??) The presentist need not posit “some secret refuge” where past things hide (and, for the spotlighter, future things as well) — no realm of ghostly individuals lacking form and substance. Of course the presentist has other sins to answer for, as shall appear when objections to the A-theory are considered, below; and some of the objections might be easier to answer if one is a growing-blocker or moving-spotlighter. But, when all is said and done, I believe presentism remains the best version of the A-theory going.

A-theory vs. B-theory: A pseudo-problem? Before considering arguments for and against the A-theory, I must address a very natural and, indeed, healthy worry that will have occurred to many by this point: Aren’t the Atheorists and B-theorists simply talking past one another? Aren’t both parties right, and simply misunderstanding one another’s claims? Whenever two philosophers disagree about some seemingly deep philosophical issue, another philosopher always seems to come along and say: “You aren’t really disagreeing with one another; if you paid more attention to the way language works, you’d see that you’re using the same words to mean different things, and talking past one another.” And occasionally the diagnosis is accurate. So we cannot ignore the “noconflict” philosophers who have attempted to deflate debates between A-theorists and Btheorists.

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I shall focus on attempts to deflate the dispute between presentists, on the one hand, and those who believe in the existence of past and future things and events, on the other.10 Here is what I take to be the nub of the presentist and non-presentist dispute.11 Both parties are metaphysicians; and, as Peter van Inwagen puts it, “The metaphysician’s aim … is to make assertions that strictly and literally describe reality”.12 Metaphor, exaggeration, and other forms of “speaking loosely” are not allowed when one knuckles down and sets forth the details of one’s metaphysical views. Furthermore, the metaphysician is interested in making claims that are unqualifiedly true; in particular, when asking the central metaphysical question, “What is there? What exists?”, there must be no unspoken narrowing of our attention to just certain kinds of things — none of what van Inwagen calls “restrictions of intended reference” (as there are when we say “There is no more beer”, meaning simply that there is no more beer in the house). Ontology is the part of metaphysics concerned with questions about what there is, or what exists. Ultimately, ontology wants to take everything into account, ignoring nothing that there is. It proceeds by asking about the “ontological commitments” of this or that theory — that is, by asking what exists, according to the theory. The non-presentist — for example, the B-theorist — thinks that, when speaking strictly and literally, and with all restrictions “off”, it is true to say that there are, or that there exist, dinosaurs and Martian outposts. The B-theory is “ontologically committed”

10 In Zimmerman, 2005, I make a case for real conflict between the B-theory and other versions of the A-

theory, as well — though perhaps the case is not quite as strong, depending as it does upon more controversial assumptions. 11 For a similar account, to which mine is indebted, see Sider, pp. 16-17. See also Zimmerman, 1998. 12 Van Inwagen, p. 3.

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to objects and events from many different times. The presentist disagrees, insisting that, speaking strictly and literally, and not restricting our interest in any way, it is false to say that there are, or that there exist, any such things. The ontological commitments of presentism are vastly fewer. The no-conflict philosopher is skeptical about whether the presentist and Btheorist really disagree about anything, and offers the following diagnosis of what is really going on. What the presentist says is true, if she is restricting her intended reference to just things that presently exist. And sometimes we do use “there are” in this restricted sense, a fact that the non-presentist should be able to recognize. After all, one can say, “There are no dinosaurs”, and speak the truth if one means simply that they do not exist now. On the other hand, what the non-presentist says can be true, too. It is obvious that there are dinosaurs in one “strict and literal” sense of “there are” — namely, when “there are” means “there are, or were, or will be”. And sometimes “There are” does mean that; for instance, in a sentence like “There are events in the history of the universe that no one will ever have witnessed, such as the Big Bang and the death of our sun.” The diagnosis of the no-conflict philosopher is beguiling. But it can be applied in many areas of metaphysics, to deflate many ontological debates; and careful examination of the general strategy should raise suspicions. The no-conflict approach to apparent disagreement seems to me to be this. Suppose philosopher X says “there are no so-andsos”, and philosopher Y says “there are so-and-sos”. The no-conflict philosopher finds a context in which it is more-or-less appropriate to say “there are so-and-sos”, and insists that this appropriateness shows that there is a meaning of “there are” that makes what Y

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says strictly and literally true. Then the no-conflict philosopher points to the possibility of restrictions in intended reference (“there is no beer” — that is, restricting our attention only to what is in the fridge, there is no beer), and claims that this explains how X’s statement (“there are no so-and-sos”) can be true as well. We are often only interested in certain so-and-sos, and X speaks truly by tacitly invoking that restriction of our interest.13 But if this line of reasoning is what makes the no-conflict philosopher’s claims seem plausible as a deflation of the debate between presentist and non-presentist, the metaphysician is bound to put up a fight. Many disputes in metaphysics are about ontology, about “what there is”, and take the form of one party denying that there are entities of some kind, and another party insisting that they exist. “Modal actualists” disagree with “modal realists” about whether there are merely possible objects — for example, whether there are such things as the merely possible brothers and sisters that I never had. The modal actualists, who deny that there are such things, surely ought to be allowed to put their (collective) foot down and say, “We mean that, with all restrictions ‘off’, there are none, period!” (Ch. 3 of this volume is a debate between a modal actualist and a modal realist.) Many other debates in metaphysics are disagreements over ontological commitments. Some philosophers deny, and others accept, that there are “creatures of fiction” (for example, Sherlock Holmes); some deny, and others accept, that there are “merely intentional objects” such as the Fountain of Youth or Zeus. But, in each of these cases, as in many others, a no-conflict philosopher could point to circumstances in which it would be more-or-less appropriate

13 Eli Hirsch’s “no-conflict” strategy is much more sophisticated (and more complicated) than the one I

consider here. See Hirsch’s contribution to Ch. 9 of this volume; and Hirsch, 2002. For criticisms of Hirsch, see Matti Eklund’s essay in Ch. 9; and also Sider, 2007.

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to say that there are things of the contested kind. “There are brothers and sisters whom I will never know, because fate prevented my parents from having any other children besides myself.” “There are fictional detectives who are more famous than any living detective.” “There are magical objects and gods that have been sought and worshipped, although they do not exist.” The philosopher who thinks that it is simply and absolutely false that there are merely possible objects, fictional characters, or imaginary things, should not be dumbfounded by the fact that it is sometimes appropriate, in certain circumstance, to say that there are such things. It may be okay to say it — one may thereby convey something more-or-less true — but one need not agree that its occasional conversational appropriateness implies that it is therefore strictly and literally true! We would not be able to coherently deny the existence of anything, if the noconflict strategy were consistently employed. For example, suppose you (quite sensibly) deny that there is such a thing as a round square (to take something that almost everyone will agree does not exist). Some no-conflict joker can come along and say: “I see; so there is at least one thing you do not believe in, namely, a round square. That is an appropriate way to report what you just told me about yourself, right? But if that is an appropriate thing to say, then it must be strictly and literally true; and therefore it is strictly and literally true to say that there is a round square. So when you said that there is no such thing as a round square, what you said could only have been true in virtue of a tacit restriction of our attention, in that moment, to just certain kinds of thing — for example, things in space and time. When I truly said, a moment ago, that there are round squares, I was simply removing that restriction.”

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How should you respond to the sophistical arguments of this mad-dog no-conflict philosopher? As follows: (i) you should deny that it is strictly and literally true that there is a thing that you do not believe in and that is a round square (rather, you should say, what is strictly and literally true is that you do not believe that there is such a thing as a round square); and (ii) you should insist that, when you say “there is no round square”, you intend that all restrictions of intended reference are “off”, and that this is absolutely true — there are no round squares whatsoever, anywhere! And if that is fair for you to say about the round square, then we presentists should be allowed to say analogous things in the face of our no-conflict critic: (i) we think that it is not strictly and literally true that there are dinosaurs; (ii) when we say “there are no dinosaurs”, we intend that all restrictions are “off”, and we mean that there are none, period. If this response on the part of the presentist is not sufficient to turn aside the criticisms of the no-conflict philosopher, many of the metaphysical debates described elsewhere in this book are empty — “full of sound and fury, but signifying nothing”.14 It is healthy, I suspect, for metaphysicians to wonder whether this is not sometimes the case. But the presentist should spend no more time agonizing over this than metaphysicians engaged in many another ontological debate.

Objections to and Reasons for Presentism and the A-theory So far, I have distinguished a range of versions of the A-theory, argued that presentism is best, and defended it as a distinctive metaphysical thesis (not the triviality alleged by the no-conflict philosopher). What remains is to assess the relative plausibility of the A14 No-conflict philosophers have attempted to deflate the debates exemplified in Chs 1, 3, 6, and 8 of this

volume, each of which turns crucially on a question of ontological commitment.

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theory and the B-theory. I begin by criticizing a couple of the most popular arguments against presentism in particular, and the A-theory in general; and I conclude with the reason I accept the A-theory. Arguments against the various versions of the A-theory fall fairly neatly into two categories: those that could just as easily have been given before Einstein’s Special or General Theory of Relativity was formulated, and those that could not. The former generally appeal to philosophical principles that are independent of the results of the scientific study of time. These principles tend to be controversial, and I usually find the arguments based upon them to be no harder to resist than the arguments against many other philosophical views that are widely admitted to be tenable. It would be impossible to discuss very many of these metaphysical arguments in a relatively brief, introductory defense of the A-theory; so I choose what is probably the most popular, which charges presentism with a disreputable lack of “truthmakers”.15 I devote much more space to the alleged conflicts between the A-theory and Relativity; if the two are as irreconcilable as many philosophers contend, the A-theory is in deep trouble.

Presentism and “Truthmakers” One of the most popular purely metaphysical arguments against presentist versions of the A-theory is based on something called “truthmaking”. It is fairly natural to suppose that, whenever someone says something, and what they said was true, then there must be something “in the world” — some real object, 15 Among the purely metaphysical arguments against the A-theory, none is more famous than McTaggart’s

“infinite regress” argument; but it is such a lousy argument (C.D. Broad rightly called it a “philosophical howler”) that it would not be worth mentioning were there not still a couple of well-known philosophers who defend it. See McTaggart, 1908, 1998; and, for criticism, Broad, 1938.

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thing, event, state of affairs, or fact — that “makes” what they said true. Philosophers have developed this idea by spelling out various “truthmaker principles”. One plausible way to affirm the need for truthmakers would go like this: For every true proposition — where a “proposition” is the sort of thing that can be believed and doubted, the sort of thing that can be true or false — there must exist something that requires that the proposition be true — in other words, a thing that could not possibly exist, unless the proposition in question is true. Frequently, the opponents of a philosophical theory will attack it by alleging that it violates this sort of truthmaker principle. Defenders of the theory are said to posit truths of some kind for which they cannot provide adequate grounding. Here’s an example of a truthmaker objection in action: Some philosophers believe that laws of nature are really just ways of summing up the facts about the powers and liabilities of the fundamental particles and fields; and that these powers and liabilities — e.g., the tendency of massive objects to attract one another, of similarly-charged things to repel one another, etc. — are “brute”; they are basic to causal explanations of why things behave the way they do, and there is no more to say about these powers other than to describe their causes and effects in various circumstances. Other philosophers are appalled by the idea of “ungrounded” powers, dispositions, or liabilities. Such properties are too “spooky”, too “hypothetical”, not sufficiently “categorical”, etc. And these philosophers are likely to raise a truthmaker complaint against the defenders of basic powers: “Point to something in the world”, they will say, “that makes it true that this

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particle has the power to repel that one, when in fact it is not doing any repelling and may never do any repelling.”16 But truthmaker objections are rather blunt instruments with which to attack someone else’s theory. For the defender of the theory may always respond by positing “brute facts” involving the problematic notion, and insist that these facts make the problematic propositions true. So, for example, the defender of brute powers and dispositions can simply say, “A disposition or power is a real feature or property of the things that have it; and just as there exist facts about the non-dispositional properties of things in virtue of which it is true to say that they have those properties, there are facts about the dispositional properties of things in virtue of which it is true to say that they have these dispositions. Where’s the problem? You want truthmakers, there are your truthmakers.” Unless the opponent can say a good deal more, specifically, about why it is wrong to take dispositions as primitive or brute features of things, the truthmaker objection amounts to little more than dissatisfied grumbling. And once those deeper objections to the very idea of primitive dispositions are on the table, one has an argument that ordinary talk about dispositions requires analysis or replacement in non-dispositional terms (or that such talk simply never conveys anything true); and there is no need to drag in the business about “truthmakers”. The truthmaker objection to presentism is similar to the complaint about ungrounded powers. When dinosaurs roamed the earth, they passed many places without leaving much of a trace; most footprints were washed away or never allowed to petrify into solid rock, for example. An opponent of presentism could make the following 16 Ch. 2 of this volume continues the long-running debate about laws of nature.

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demand about any one of these places: “Point to something in the world”, the objector says, “that makes it true that a dinosaur walked past this place 150,000,000 years ago. It is true, but there is nothing about the way the world is now that requires that it be true or that makes it true; and according to you presentists, there is nothing more to the world than the way it is now. So you have no truthmakers for such straightforward truths about the past.” One presentist response mirrors the response of the defender of brute powers: “There are ‘backward-looking’ properties that objects really have, properties like having been occupied by a dinosaur 150,000,000 years ago; and there are real facts about which objects have these properties, facts that make propositions about the past true. What’s the problem?” Although the opponents of presentism have attempted to answer this challenge; by my lights, they are still in the dissatisfied grumbling stage.

Relativity vs. the A-theory I have only really lost sleep over one objection to the A-theory: that it is in conflict with some of the most well-confirmed scientific theories we have, namely, Einstein’s Special and General Theories of Relativity. If there is a real conflict, then this is a very serious objection indeed. Philosophers have frequently given arguments to the effect that space or time must have a certain structure; for instance, that the geometrical structure of space must be Euclidean. Working scientists, meanwhile, have usually ignored them and calmly gone about their business, eventually proving conclusively that space or time doesn’t have that structure. The canny A-theorist should be very reluctant to go down this road.

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But what exactly is the nature of the supposed conflict? Why is it harder to combine the A-theory with Relativity than with a Newtonian conception of time, say? Answering this question requires a detailed comparison of Newton’s theory of “absolute” space and time with the radically different conception found in Einsteinian Relativity. Theories of motion (kinematics, which ignores the forces acting on bodies, and dynamics, which takes them into account) are most perspicuously expressed in terms of a “space-time manifold”. In one sense of the word, a manifold is any set of things that are interrelated in such a way that their structure can be described geometrically. A spacetime manifold is a set of minimal-sized locations in space and time, “points” at which something could happen or be located. Not every theory that can be stated in terms of a manifold really demands that we believe in space-time as an existing thing, an extra part of the furniture of the world — a sort of invisible jell-o filling up the otherwise empty spaces between objects (and suffusing their insides as well). But the theories of motion to be contrasted here — Newton’s and Einstein’s — appeal to structural features internal to space-time itself, structure that plays a crucial role in their explanations of why objects move as they do. So I shall assume that the manifolds described by the theories would have to be real entities. Here is a recipe for the idea of a space-time manifold in an infinitely large universe. First, imagine a set of points that constitute an infinitely large threedimensional Euclidean space. This set of points represents “the-three-dimensionaluniverse-at-an-instant”, all the locations in space at a given moment, whether or not there is anything going on in that location at that time. Now imagine an infinity of these threedimensional spaces, each representing the universe at a different instant; and suppose that

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they are all “lined up” on a continuum in an extra dimension — “the fourth dimension”, a temporal one, consisting of the set of all instants stretching from the distant past into the distant future. Within this four-dimensional manifold, there is a distinct threedimensional space for each moment; so “you can’t go home again” — in fact, you can’t even stay home, if that means staying in the same three-dimensional spatial region of the manifold for more than an instant. Various debates about the nature of space and time can be turned into disagreements about the structure of the space-time manifold. Is each three-dimensional universe-at-an-instant really infinitely large, as Newton thought? 17 Is the manifold finitely long in the temporal direction, with a beginning and end? Does it have a beginning but no end, as now seems likely? Interesting questions…but there is another sort of structure that Newton posited, and later scientists have rejected, that will be of interest here. It has less to do with the overall shape of the manifold, more to do with its internal nature on the smallest scale. Different theories about the nature of space-time say different things about the kind of structure the manifold contains — they describe its parts as interrelated in different ways. A crucial part of the manifold’s structure is metrical. Metrical structure is what makes the manifold measurable, and can be thought of as the sum total of all the fundamental distance relations holding among its points. On the Newtonian conception of space-time, there are metrical facts about the spatial distance relations among the

17 I write as though Newton himself posited a four-dimensional space-time, but this is something of a

cheat. Newton’s absolute space was a three-dimensional thing, persisting through time. Relativity requires a four-dimensional manifold, however; and it is pedagogically useful to first describe a non-relativistic four-dimensional space-time that includes Newton’s relation of being “the same place in absolute space”, before trying to describe relativistic space-time itself.

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points in a single three-dimensional universe-at-an-instant. There are metrical facts about the temporal distances between the points in any two of these three-dimensional spaces. But Newton posited a further kind of metrical structure: objective, fundamental relations of spatial distance between points that are located at different times. On his view, there is a single right answer to the question: What is the distance between this point, at this time, and that point, at that other time? The answer might be one mile, or one inch, or 100,000 miles. But the answer might also be zero, in which case the two points represent the very same location at different times. Newton’s notion of “absolute” sameness of place over time may be contrasted with that of merely relative sameness of place. Suppose I forget my book on a train, and return to find it in the very same place I left it, relative to the parts of the train (it is still there on my seat). If the train has been traveling in the meantime, the book is in a different place relative to the surface of the earth (it was in New York but is now in New Jersey). Newton held that, in addition to all such merely relative relations of same-placeat-a-different-time, there was a non-relative, objective relation of same-place-at-adifferent-time, a relation that was built into the structure of space-time itself. Other theories about the metrical structure of space-time — including Relativity — deny that the points of space-time stand in such relations. We now have just enough information about the nature of Newtonian space-time to begin to see why some have thought that its rejection, in favor of relativistic spacetime, should count against the A-theory. The A-theorist believes that there is an objective fact about what is happening at present; so she believes that there is a fact about which portion of the manifold is “lit up”

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or filled with real “happenings”. Since she naturally thinks that the A-theory has always been and will always be true, she must think that there has always been and will always be a fact of this sort — continuously changing, of course. So the A-theory requires that the manifold come with a built-in division into a series of slices, each corresponding to the-universe-at-an-instant. This is a kind of built-in structure that the Newtonian already posits. Pretend that space-time is a four-dimensional salami. Both the A-theorist and Newton believe that the salami comes from the butcher’s shop “pre-cut” into special “slices”, each of which constitutes the-universe-at-an-instant. Both agree that slicing the salami at an angle relative to the angle used by the butcher would be to make a hash of it. So the Newtonian manifold appears tailor-made for the A-theory. But Newton’s theory is no longer the best theory of motion we have. Special and General Relativity, like Newtonian physics, make ineliminable reference to a manifold with a built-in structure that plays a big role in explaining why objects move as they do. The structure they attribute to the manifold is, however, very different from that of Newtonian space-time. The structure of the Newtonian manifold, as I described it, is based upon three fundamental types of relations among points: (i) spatial distance relations within each momentary three-dimensional space, (ii) temporal distance relations between the points in different spaces, and (iii) a “same-place-at-a-different-time” relation between points in different momentary spaces. Relativity denies that (i) and (ii) are fundamental, and rejects (iii) altogether. Special Relativity (upon which I will focus) bases the structure of space-time upon a very different relation of “space-time distance”; and the picture that emerges is hard to describe. Points can be separated by positive, negative, and null space-time distances; and the meanings of these measurable quantities

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are closely tied to the explanations of motion that Relativity affords. Here are some of those connections. Relations of space-time distance give a sense to “straight lines” in the manifold: a straight line in the space is the shortest distance between two points. What is Relativity saying about points when it says they lie along a straight line and stand in positive distance relations? Such a line has a “time-like” direction; and it corresponds to the possible path of a particle that is neither accelerating nor decelerating — an object in a state of “inertial motion”. What does it mean, in Relativity, to say that points within the manifold are at zero distance from one another? Not that they are “the same place” or “the same point”. It means that they correspond to points along a path that light would take in a vacuum. What does it mean, in Relativity, to say that points are on a straight line and standing in negative space-time distance relations? In that case, the line is “space-like”; it corresponds to a straight line in a three-dimensional region of the manifold — a region that looks like the-universe-at-an-instant. But, what is crucial for present purposes is the fact that the metrical structure described by Relativity does not privilege just one set of space-like divisions of the manifold into instantaneous three-dimensional regions. According to Relativity (both the Special and General theories), there are infinitely many ways to divide the fourdimensional manifold into a series of slices, each slice corresponding to a threedimensional space that looks for all the world like the-universe-at-an-instant. These slices are three-dimensional, spatial regions cutting across the whole of the universe, and instantaneous in temporal length.

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To return to the deli-counter metaphor: Relativity says that you buy the salami “whole”. When you take it home, you are free to slice it at any angle within 45 degrees of the vertical, and the result will be slices that are just as “good” as slices resulting from cutting at any other angle within that range. (And do not suppose that there is one angle that corresponds to “perfectly vertical” slices; that is a misleading part of the metaphor.)18 The move from Newtonian to Relativistic space-time has seemed to some to provide the materials for a straightforward and devastating argument against any version of the A-theory, along the following lines. Relativity does not imply that there is a privileged way of slicing the four-dimensional manifold into instantaneous threedimensional spaces. But the A-theory does posit a privileged way of slicing the manifold. So the A-theory — a metaphysical hypothesis about the nature of time — conflicts with well-established physical theory. And history suggests that in such cases the metaphysical hypothesis inevitably loses. If this is all there is to the argument against the A-theory, I am not much impressed. Its form is: Physics does not imply that one out of some class of things has a special status; so, for a person who has learned the relevant physics, there is no reason to believe that one of them is special. The problem with this style of argument should be fairly obvious: there may be other reasons for thinking one member of the class is

18 General Relativity allows for space-time to take on all kinds of bizarre shapes (for example, it does not

rule out the possibility of “closed time-like curves” — paths through space-time that loop back upon themselves); and some of the more bizarre ones do not submit to a natural, exhaustive “slicing” into times. Physicists sometimes dismiss such models as not “physically real” (by which they mean more than just that they do not describe the actual world); and I should think the A-theorist may also reject them as “not genuinely possible ways for time to be” without too many qualms. I shall ignore them here; but the Atheorist must say something about these alleged possibilities.

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special, and no particular reason to think that if there is a difference, it will be physics’ job to discern it. I offer four inappropriate applications of this style of argument. I take them to be quite analogous to its use against the A-theory: The modal realist — a character mentioned earlier, and represented in this volume by Phillip Bricker — believes in all sorts of space-time universes that are spatially and temporally unrelated to this one. Now, we can all certainly contemplate such “other universes”, as abstract possibilities. And physics alone will not tell us whether they exist — at least, I can see no argument from statements in the language of physics, describing the contents of our universe and its laws, to the conclusion that merely possible worlds are not universes much like ours but merely spatially and temporally disconnected from us. Nevertheless, I believe that I remain fully justified in maintaining my conviction that this universe is special — that it is radically unlike the merely possible ones, if there are any such things. This belief is something that I quite reasonably take for granted; and the fact that it finds no support from physics is quite irrelevant. A second example: I believe that at least some collections of particles form larger wholes, including “macroscopic objects”, things I see and sometimes bump into. For instance, there is a collection of particles clumped together in front of me right now, particles that make up something more than just a swarm or cloud; they constitute a larger thing, namely, my laptop. But physics does not distinguish between collections of particles that form larger wholes and collections that do not; in fact, it does not, in any straightforward way, imply either that there are or are not any physical objects larger than the most fundamental particles. But belief in macroscopic objects is, I would insist, highly reasonable, and not impugned in the least by its lack of support from physics.

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A third and more controversial example involving normative properties: Some physically describable situations are good (for example, situations in which people are happy, or virtuous), and others bad (for example, situations in which people are in pain, or displaying vices); but one will search high and low for a corresponding natural distinction between these situations that can be stated in terms of fundamental physics. Should that undermine my belief in the objective goodness and badness of things? No. Finally, closer to home, I note that a similar argument against the A-theory could just as well have been made in the Newtonian context: Newtonian physics does not tell us which slice of the manifold is present; so, if the A-theory says that one of them is present, the A-theory is in conflict with physics and must be wrong. But we should not find this argument very impressive. Consider all the eternally true statements about the distribution of matter in space-time — statements that use present tense verbs like “is” to mean “is, at some time or other”, and that make no explicit mention of what is present, past, or future. Of course these “tenseless” statements do not tell us what time it is now; but who would have thought they should? Now add to these statements the laws of Newtonian physics. The argument from Newtonian physics against the A-theory would be that this combination provides a complete description of our world in terms of fundamental physics, and yet it fails to distinguish between past, present, and future times; so no such distinction exists. But surely this is a lousy argument. The point of physics is to find laws that can remain true always and everywhere. Is it any surprise that simply stating such laws, in conjunction with statements that are neutral about the present time, does not suffice to specify the present time?

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A “tenseless” physical description plus laws can be compared to a “tenseless” historical description plus “social laws”: Suppose a history of the human race were written very carefully, so as not to reveal anything about when the author lived; and, in the final chapter, the author added some eternally true “laws” about the development and decline of societies or empires. Would the fact that a history of that sort could be very accurate show that there was no objective fact of the matter about what empires are past, and which ones are present? No. The author was being careful to weed out any statements that might imply such facts; we should not be surprised when they are absent. So I deny that there is a simple path from “physics does not imply a distinguished slicing” to “any theory that does is false”. The critic of the A-theory is pointing out that relativistic physics — when its description of the world is carefully purged of all reference to present, past, and future — fails to settle which slice of the manifold is present, and which slices were and will be present. But it also fails to settle a lot of other things about which I have plenty of convictions — for instance, whether every possible world exists as a genuine universe, and whether there are composite objects or intrinsically good things. I do not see any special reason to expect that, if there were facts about what parts of the manifold are present (and facts about what past and future parts were or will be present), they would be expressible in a “tenseless” language using only terms from fundamental physics. There is, however, a more serious worry about combining the A-theory with (Special or General) Relativity: namely, that it would force us to reject Relativity in favor of a theory of motion like Lorentz’s — a theory according to which the manifold of space-time has Newtonian metrical structure and there are forces that are “hidden” from

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us in a strangely conspiratorial way. If this is what the A-theory requires, it would be deeply revisionary. Lorentzian space-time is not just relativistic space-time plus some extra facts about a privileged slicing. Lorentz rejects the structure that Relativity posits, along with the explanations of motion made possible by that structure. Again, some background is required: namely, the path that led to Lorentz’s view. Lorentz knew that, when a flash of light is generated at a certain location, it spreads through space in a way that does not depend upon the speed of its source. Contrast a flash of light to a bomb. If a bomb explodes while moving rapidly toward me, sending shrapnel in all directions, the bits coming toward me will have a higher velocity than the bits moving away from me. The shrapnel from a stationary bomb exploding at the same place would, on the other hand, have the same velocity in all directions. But a flash of light from a moving bulb and a similar flash from a stationary bulb would not differ in this way. Each flash would spread out from the point of origin in the same way — rather like the waves from a pebble dropped in still water. Now, in the case of the pebbles and waves, only people in boats that are stationary relative to the water will find that all the waves are moving at a constant rate away from their point of origin. Those in a stationary boat north of the place where the pebble was dropped will find the northgoing wave front coming towards them just as quickly as the south-going wave front is moving away from them. But compare such a boat with one that is moving from the north toward the source of the waves, and that has the same velocity, relative to the water, as the waves themselves. People in this boat will find the north-going waves striking their bow more rapidly; and they will find that the south-going waves directly

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opposite them are stationary relative to their boat — the south-going waves and the south-going boat maintain a constant distance. Prior to the formulation of Relativity, scientists naturally assumed that light and objects in motion would behave much like waves and boats. There would be just one state of motion that a laboratory could be in that would allow its instruments to measure the rate of propagation of light as being the same in all directions. If light were a wave propagated through a medium of some sort, the objects in such a laboratory would be stationary relative to that medium. The measuring devices in other laboratories, in motion relative to the truly stationary laboratory, were expected to measure the speed of light as higher in some directions than others. The objects in the stationary laboratory — together with any other objects, anywhere in the universe, that were stationary relative to them — should constitute a privileged “frame of reference”. If a thing were in the same position at different times, relative to these objects, then it would have a special status. In this way, the behavior of light was expected to privilege just one relation of “same-placeat-a-different-time”; and space-time would come with the sort of built-in structure that Newton posited. It came as quite a shock when scientists discovered that, no matter one’s state of motion, light still appeared to propagate at a constant rate in all directions. What could this mean? Lorentz believed that the appearance must be misleading; and he supposed it was caused by fluctuations in the sizes of measuring devices and in the behavior of clocks when they are in motion relative to objects in the privileged frame — the set of objects relative to which the speed of light is truly constant in all directions. So how come the equipment in a laboratory moving rapidly (relative to the privileged frame of

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reference) toward some light source does not measure the speed of light as faster than the equipment in a laboratory at rest in the privileged frame? Lorentz’s hypothesis was that the clocks in the moving laboratory have slowed down and the distances among the parts of its instruments have shrunk. The clocks and instruments in the stationary laboratory do not undergo these strange effects. But there is no way to tell which lab one is in; the contractions and time dilations seem precisely designed to conceal this information. Relativity provides a very different explanation of the apparent constancy of the speed of light in all frames of reference. The details can be found in any introduction to Relativity; but one important feature of the theory is that it does not have to posit strange forces that speed up clocks and shrink instruments in some laboratories but not others. Lorentz’s theory and Relativity also differ in the way they explain the motions of things that move at less than light speed. Suppose you are moving rapidly away from me; and suppose we are both moving inertially, and isolated from outside influences. The trajectory you will appear to me to take in the future is, according to Lorentz, a function of your past velocity relative to the privileged frame and my past velocity relative to that frame, plus whatever forces are responsible for shrinking objects and slowing clocks (and other processes) in things that are in motion relative to the privileged frame. Relativity, on the other hand, explains our motion in terms of the straightness of our space-time paths and their places within the manifold, not in terms of our velocities relative to a privileged frame of reference. As unaccelarated objects, we have been traveling on “straight lines” in space-time; in the absence of forces, we will go on to occupy points along the same lines; and these lines are sharply divergent. Here, the structure of space-time (its straight lines and angles) is doing all the work of explaining

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why the objects do what they do. (If there were other forces at work, if we or the other object were undergoing acceleration or deceleration, more factors would come into play.) The Lorentzian explanation, on the other hand, appeals to both the metrical structure of space-time (not the relativistic structure based on space-time distances, but the NeoNewtonian structure based on more traditional spatial and temporal relations) and rather mysterious forces that produce spatial contractions and temporal dilations. It should be obvious that Lorentz’s theory of motion and Relativity provide radically different explanations of the observed facts. They appeal to different kinds of structure within the space-time manifold; and Lorentz’s theory has to posit various “invisible” forces and effects, ones that are unknown to Relativity. For many reasons, Lorentz’s theory is currently highly unpopular (to put it mildly). If the A-theory implied that space-time does not have the structure Relativity uses to explain states of motion, but must instead have the Newtonian structure of Lorentz’s space-time, the A-theory would be in serious danger of refutation by physics. Relativistic explanations of the motions of objects have many theoretical advantages over Lorentzian explanations (though they make the same observable predictions). If these advantages are sufficiently great, they serve as important evidence against any theory that requires a return to Lorentz. Accepting the A-theory implies that there is something special about one way of slicing the manifold. Fortunately, the additional temporal structure does not automatically lead to a Lorentzian theory of space-time. Relativistic explanations of motion are not vitiated simply by privileging one way of slicing the manifold into timelike slices. The A-theorist need not deny that the metrical facts based on relativistic

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space-time distances are fundamental features of the manifold, and fundamental to explaining why particles and photons move as they do. Consider a different reason to suppose there is something special about one way of slicing the manifold — a benign sort of extra temporal structure that our world probably in fact exhibits. There is evidence that one way of slicing the space-time manifold is physically special, distinguished from others in virtue of constituting a kind of “cosmic time”. If the universe is as homogeneous as it appears to be, eternally existing “clocks” moving with the ever-expanding galactic clusters would stay synchronized with one another. Had it been radically inhomogeneous, such synchronization would not have been possible. If our universe allows for the keeping of cosmic time, there is a “cosmictime slicing” of the four-dimensional space-time manifold: the slicing that judges simultaneity in agreement with the clocks — or, with what such clocks would have read, had they existed. These slices are special, albeit not intrinsically special. Still, they are distinguished by a physically interesting feature of the universe, a feature that other sets of slices lack. No one thinks that a theory of the structure of our cosmos that admits cosmic time is in conflict with Relativity. Why not? Because simply recognizing a physically distinguished way of slicing does not automatically privilege the properties that objects have relative to that slicing, nor does it privilege the metric properties of the slices of the manifold that are part of that way of cutting it up. One can admit that there is something a little bit special about the distances between objects and the time periods between events, according to the cosmic time slicing, without having to suppose that these distances and periods play a fundamental role in the laws of motion. One might even posit some more robust phenomenon associated with cosmic time — there might even be causal laws that mention simultaneity-relative-to-cosmic-time — yet not reject Relativity. 34

For example, suppose someone thought that ghosts or angels are located in space, and able to communicate telepathically with one another (but not with us) at speeds up to, but not including, instantaneous-relative-to-cosmic-time. If the imagined extra laws about telepathy imply nothing about the way particles move around, but only describe how ghosts and angels think, then the causally important additional structure could in no way undermine the Relativistic explanations of physical phenomena. Perhaps any faster-thanlight causation would violate the letter of Relativity (depending upon how one formulates the theory); but superluminal angelic or ghostly telepathy would not constitute a revolution in the theory of motion. Now comes the $20,000 question: Is the privileged slicing posited by the Atheorist more like that of cosmic time or that of Lorentz’s theory? Does the A-theorist’s privileging of a way of slicing the manifold automatically undermine Relativity’s explanations of physical phenomena, introducing a different fundamental structure? Or can it leave the relativistic web of space-time distance relations intact and still doing the job it was intended to do? The A-theorist need not suppose that there is anything physically special about the privileged slicing that she posits. In that case, the added structure seems much more like cosmic time than a Lorentzian return-to-Newton. According to this sort of A-theory, physics simply leaves it undetermined which of the many ways of slicing the manifold corresponds to the genuine “march of time”. And there is good reason to doubt whether the extra structure posited by the A-theorist would be relevant to laws of motion. If the fundamental laws of motion are statements about the ways in which a particle-at-a-point will tend to move, given its past location and the forces acting upon it, and if Relativity is right that the speed of light is an upper limit on how quickly an influence can reach a given point, then nature does not “care” what is going on in parts of the manifold that could only be connected to the point by faster-than-light paths. And the A-theorist’s extra facts about a privileged way of slicing are, after all, facts about what goes on in

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those parts of the manifold. So these extra facts are irrelevant to laws of motion that take this form. On the other hand, an A-theorist might have reason to suppose that there are special physical phenomena associated with her privileged slicing of the manifold into co-present sets of points. One school of quantum physicists asserts that there must be faster-than-light influences connecting the “entangled” particles described by quantum mechanics. Suppose they are right. The A-theorist might well have reason to suppose that the upper limit of such influences is the sets of events that are simultaneous according to her privileged way of dividing the manifold. An A-theorist would need to look closely at the quantum phenomena, and also carefully consider the metaphysical difference she posits between past, present, and future, before deciding whether to accept some connection between the two reasons to introduce extra space-time structure. But if it seems likely to her that the two coincide, what is the result? She is not being scientifically revisionary — since, by hypothesis, there is scientific reason (from quantum theory) to regard one way of slicing the manifold as special. If she ends up concluding that the two ways of slicing do not coincide, then the additional structure she posits is simply a further fact about the manifold, in addition to the special facts posited by a theory explaining quantum entanglement. Either way, she is not removing the space-time structure posited by Relativity for disreputable metaphysical reasons.

Presentism and the Manifold So far, I have been writing as though the version of the A-theory to be reconciled with Relativity is one that accepts the eternal existence of a four-dimensional manifold of space-time points, including ones that are in the past and future. Does Relativity require 36

acceptance of such an entity? Is the theory ontologically committed to the existence of the entire manifold? And if so, doesn’t this show that at least the presentist version of the A-theory is incompatible with Relativity, since the presentist denies the existence of past and future things — including, one would have thought, past and future points of spacetime? I do not see that the pull I feel toward presentism has much to do one way or the other with ontological commitment to the structured four-dimensional manifold of points described by Relativity. I believe that there exist only a relatively few events and objects. I exist, and the sounds I am hearing; but there is no such thing as the Peloponnesian War or Alexander-the-Great’s horse, Bucephalus; nor is there such a thing as the first manned Martian landing or my first great-grandchild. But suppose I admit that there are empty space-time points that were filled with things and events, and other empty space-time points that will be filled with things and events — a manifold of which only one threedimensional slice is occupied. Suppose I posit this four-dimensional manifold because physics seems to require that there be a manifold with such-and-such properties — that is, because interactions among objects alone are not enough to explain why things move as they do. Should this bother me, as a presentist? I think not. Notice that this manifold is a theoretical posit, something no one would have believed in were it not for its role in a scientific theory. An A-theorist, like everyone else, should look to science for information about the structure of such things. My convictions about the unreality of past and future objects and events, on the other hand, are convictions about horses and wars and people; they have little to do with

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questions about what sorts of theoretical entities should be allowed to figure in our best scientific theories. How should a presentist think about the space-time distance relations between points that underwrite the structure of the relativistic four-dimensional manifold? The relations that are most important for causal purposes are positive and null space-time distance. They underlie space-time’s role in telling particles and photons “what to do next.” Where will a photon go if it is located at a certain point (and in a vacuum)? The sets of points that are on light-like paths (points each of which is at zero space-time distance from the others) tell a photon located at one of them to “stay on this line, in future”. Where will a particle go if it is located at a certain point and moving inertially? The relativistic structure of the manifold implies that some points constitute “straight lines” with positive distance relations; their standing in this relation means that they tell a particle located at one of them, and not acted upon by any forces, to “stay on this line, in future”. One might say that the points at zero-distance are connected by a relation of “light-like accessibility”; and that the straight lines with positive distances among their points are connected by a relation of “inertial accessibility”. It is less clear to me how a presentist should think about space-time distance relations holding between “space-like separated” points (distances represented as negative numbers in most presentations of Special Relativity). I see no obstacle, in principle, to a simple approach: they are spatial distances among sets of points that constitute three-dimensional spaces. One of the surprising upshots of Relativity, no matter one’s metaphysics, is that an instantaneous point can be part of more than one instantaneous, three-dimensional space.

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An A-theorist might suppose that the manifold has always existed, and has always had all the structure it needs in order to play its part in explaining why the matter occupying it moved, moves, or will move this way rather than that way. On the other hand, the A-theorist might suppose that the manifold has been “growing” with the progress of objects and events through it; or that all its points have always existed, but have only acquired their precise relations of space-time distance when the combination of laws, matter distribution, and already-determinate-manifold-structure was sufficient to determine the shape of future parts of the manifold. In any case, so long as only one time-like slice of the manifold is filled with horses and headaches and the like, I shall have everything that a presentist should care to preserve in an A-theory.

Why Think the A-theory Is True? It would take a book, and more expertise than I possess, to fully and convincingly respond to the objections from Relativity sketched above; and several more books to respond to the plethora of other objections B-theorists have raised over the years. I can do no more here than to record my conviction that the prospects for such responses seem to me to be good. I conclude my defense of the A-theory by explaining why it is that I accept it. My reason for believing the A-theory is utterly banal (some philosophers reading this essay will want to say “insipid”): It is simply part of commonsense that the past and future are less real than the present; that the difference between events and things that exist at present, and ones that do not, goes much deeper than the difference between events and things near where I am and ones that are spatially far away — in Australia, for

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example. These platitudes about past, present, and future may not immediately imply one particular version of the A-theory; and perhaps no very specific metaphysical theory deserves to be called “the” commonsense view about time. But the various versions of the A-theory — at least, presentism and the most plausible versions of the growing-block and moving-spotlight theories — are metaphysical pictures that preserve this commonsense conviction. And the B-theory does not. J. J. C. Smart is a B-theorist who is particularly forthright in his rejection of the platitudinous beliefs that imply the A-theory. The source of these beliefs is, he thinks, a kind of misguided “anthropocentrism” — though perhaps “egocentrism” would be just as appropriate a label for what Smart has in mind.19 At each moment, I am, quite naturally, especially interested in the place in time that I happen to occupy right then; and, from this special interest, I slide into the false conviction that my place in time is also objectively more important than the others. According to Smart, when you become a B-theorist, you adopt a more accurate perspective: you come to recognize that this particular time is no different than any other — it merely happens to be more important to you while you are located at it. In this section, I first explain what I mean by saying that the A-theorist’s favorite platitudes about the deep difference between past, present, and future are “part of commonsense”. And then I say why their having this status should be thought to count in their favor. It would be a mistake to think of “commonsense” as a faculty, in the full sense of the word. It is not a special source of human knowledge — a distinctive and innate method by means of which we human beings gather information about the world, like

19 See Smart, pp. 132-42.

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sight or hearing. What it is for some statement to be commonsensical is just for it to seem obviously true to most sane human beings; for it to be part of the stock of things we all — or at least almost all of us — take for granted. Is the A-theory part of commonsense? I think so. Everybody knows that when events and things “recede into the past”, they are very different from the way they are when present; and that the future is a “realm of mere possibilities, not realities”. These are truisms denied by a relatively small group of people — basically, people who have become accustomed to using spatial metaphors to understand temporal notions (as one does when drawing space-time diagrams or reading the more consistent science fiction stories about time travel). Still, how does the ubiquity of belief in the A-theory constitute serious support for the view? Haven’t we discovered that, at various times in our history, the vast majority of us have taken things to be obvious that turned out to be provably false — for instance, that the sun rotates around the earth? Why think the A-theory is any better? Of course it’s true that a great many things that have seemed obviously true to most people have turned out, upon inspection, to be false. All I insist upon, however, is that something’s being commonsensical must be allowed to count very strongly in its favor, other things’ being equal. Those who deny this relatively modest role to commonsense are courting extreme skepticism about much more than just the A-theory. Epistemology is the philosophical examination of knowledge and related notions, such as evidence and rationality. One task of epistemology is to explore the question each of us can put by asking, “What is it reasonable for me to believe?” Epistemologists have learned something from the failure of modern attempts, from Descartes onward, to find absolute certainties and infallible chains of reasoning to take us from certainties to the rest of the things we think it is reasonable for us to believe. The moral is this: Unless

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we are to become extreme skeptics, we must allow that it is reasonable to believe things that seem obviously true, in the absence of special reasons to doubt them; and we must allow this even if the beliefs are admittedly not certainties, and cannot be proven in any interesting sense. This conclusion privileges commonsensical beliefs. To be part of commonsense, a thing must seem obvious to almost everyone; so, for almost everyone, it is reasonable to believe it — in the absence of serious objections. One might say it is “innocent until proven guilty.” Now, one reason to doubt the truth of some seemingly obvious statement would be the fact that it is disbelieved by many other people whom one takes to be equally well-placed to have an opinion on the matter. But if some particular example of an obvious truism is part of commonsense, then it is widely held; and so, assuming most people are well-placed to have an opinion about it, it has passed a further test of reasonableness. For a belief to be part of commonsense, is, then, a considerable “epistemic boon” or “plus”. Those who accept the commonsensical platitudes about the past, present, and future, are, therefore, at least justified in regarding the A-theory as “innocent until proven guilty”. I take it to be as innocent, at least on first pass, as many other commonsensical beliefs that skeptical philosophers have tried to undermine: for example, the belief that there is a world of objects “outside” my mind, not dependent upon anyone’s awareness of them for their existence; the belief that others have experiences much like mine, and are not mere “zombies”; and the belief that we can reach justified conclusions by relying upon induction (an example of “relying upon induction” is postulating laws of nature that are universally true on the basis of a limited number of observations confirming the laws). All these commonsensical beliefs (and many more besides) have been challenged by skeptics; and their arguments often have a certain plausibility — that is, their weakest premises are philosophical statements that sound at least moderately reasonable. Most intelligent adults, when confronted by the cleverest skeptical arguments for the first time, do not immediately see the best way to resist them. (Indeed, in some cases, there is

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considerable disagreement about how to resist them even among philosophers; and widespread agreement only that, one way or another, the skeptical arguments must be unsound.) When typical skeptical arguments are brought to bear upon beliefs as innocent as these targets, it is more sensible to conclude that there is something wrong with the premises (even if one cannot quite put one’s finger on it) than to accept their conclusions. And it is more reasonable to reject the least plausible premise of these skeptical arguments on the basis of the falsehood of the conclusion — even if that premise seemed true, when one first heard it — than to become a skeptic. Why? Because the conclusion is the rejection of something that is part of commonsense — something that all but a few of us take to be utterly obvious. So, unless we are prepared to become skeptics, we should admit that being part of commonsense gives a belief a non-negligible positive status. Such beliefs are not something that skeptical philosophers should expect us to give up at the merest hint of controversy, or in the face of anything but a powerful case against them. Everything turns, then, on positive arguments against the A-theory. I was not much impressed with the truthmaker objection against presentism; and I can here only report that other metaphysical arguments against the A-theory in general and presentism in particular leave me unperturbed, though sometimes not sure exactly what to say. Skepticism on the basis of these arguments seems to me to be no more warranted than in many other cases in which we side with commonsense, while not being entirely certain what is wrong with the skeptic’s argument. Arguments from Relativity are more troubling, mainly because the progress of science has taught us to be extremely wary of putting much stock in the details of our untutored judgments about cause and effect, and the laws of nature. We have ample evidence that, in these matters, not everyone is equally well-placed to know the truth; and in that case, we should listen to the experts, not the deliverances of commonsense. Now, the scientific experts support Relativity (though not unequivocally, given the difficulties

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of unification with quantum mechanics). Fair enough. But, as I’ve tried to show, the arguments that are supposed to lead from Relativity to the falsehood of the A-theory are not very impressive. I find that, like most people throughout history, I believe things that imply the Atheory; indeed, affirmative answers to (i) and (ii), the questions with which we began, seem obviously right. It is reasonable to believe what seems obvious; the commonsensical is “innocent until proven guilty”. If I am right to doubt the force of arguments against the A-theory from metaphysical principles and Relativity, the innocence of the A-theory remains relatively unsullied. I would not claim to know that that the A-theory is true; but few philosophers should claim to know the substantive philosophical doctrines they defend against their equally intelligent and well-informed philosophical opponents. I do insist that it remains reasonable for me to believe it. And that is usually the best one can say for a metaphysical theory.20

20 I am grateful to Frank Arntzenius, John Hawthorne, Tim Maudlin, Ted Sider, and Jason Turner for

discussions in which I learned (or tried to learn) a great deal about Relativity and the ways it might conflict with presentism. Frank and Ted, in particular, went above and beyond the call of collegial duty. None of them (with the possible exception of Ted, who provided lots of help with late drafts) should be held responsible for whatever confusions remain in this paper.

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REFERENCES Adams, Robert M. 1986. “Time and Thisness”, in Peter A. French, Theodore E. Uehling, and Howard K. Wettstein (eds), Midwest Studies in Philosophy: Volume 11 (Minneapolis: University of Minnesota): 315-329 Augustine. 1961. Confessions, trans. by R. S. Pine-Coffin (Harmondsworth, U.K.: Penguin) Braddon-Mitchell, David. 2004. “How Do We Know it is Now Now?”, Analysis 64: 199-203 Broad, C. D. 1923. Scientific Thought (London: Routledge & Kegan Paul) Broad, C. D. 1998. McTaggart’s Arguments against the Reality of Time: an Excerpt from Examination of McTaggart’s Philosophy, in van Inwagen and Zimmerman (eds), 1998: 74-79 (also to appear as Ch. 10 of van Inwagen and Zimmerman (eds), 2007) Forrest, Peter. 2006. “General Facts, Physical Necessity, and the Metaphysics of Time”, in Zimmerman (ed.), 2006: 137-152 Hirsch, Eli. 2002. “Against Revisionary Ontology.” Philosophical Topics 30: 103–127 McTaggart, J. McT. E. 1908. “The Unreality of Time”, Mind 17: 457-74 McTaggart, J. McT. E. 1998. “Time: An Excerpt From The Nature of Existence”, in van Inwagen and Zimmerman (eds), 1998: 67-74 (also to appear as Ch. 9 of van Inwagen and Zimmerman (eds), 2007) Merricks, Trenton. 2006. “Goodbye Growing Block”, in Zimmerman (ed.), 2006: 103110

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Prior, Arthur. 1996. “Some Free Thinking About Time”, in Jack Copeland (ed.), Logic and Reality (Oxford: Clarendon Press), pp. 47-51; reprinted as Ch. 14 of van Inwagen and Zimmerman, 2007. Sider, Theodore. 2001. Four-Dimensionalism (New York: Oxford University Press) Sider, Theodore. 2007. “Ontological Realism”, in David Chalmers, David Manley and Ryan Wasserman (eds), Metametaphysics (Oxford: Oxford University Press) Smart, J. J. C. 1963. Philosophy and Scientific Realism (London: Routledge and Kegan Paul) Smith, Quentin. 1993. Language and Time (New York: Oxford University Press) van Inwagen, Peter. 1998. “Introduction: What Is Metaphysics?”, in van Inwagen and Zimmerman, (eds), 1998: 1-13 van Inwagen, Peter and Dean Zimmerman (eds). 1998. Metaphysics: The Big Questions (Malden, Mass.: Blackwell) van Inwagen, Peter and Dean Zimmerman (eds). 2007. Metaphysics: The Big Questions, 2nd ed. (Malden, Mass.: Blackwell) Williamson, Timothy. 1999. “Existence and Contingency”, Proceedings of the Aristotelian Society, Suppl. Vol. 73: 181-203 Zimmerman, Dean. 1998. “Temporary Intrinsics and Presentism”, in van Inwagen and Zimmerman (eds), 1998: 206-219 Zimmerman, Dean. 2005. “The A-theory of Time, the B-theory of Time, and ‘Taking Tense Seriously’”, Dialectica, Vol. 59: 401-457 Zimmerman, Dean (ed.). 2006. Oxford Studies in Metaphysics, Vol. 2 (Oxford: Oxford University Press)

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