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Does the future exist? In theory, is the future a 'place' that I can go to in a

Does the future exist? In theory, is the future a 'place' that I can go to in a time-machine or does the universe alter in such a way that my desired era appears before me?

A timely topic, if you'll pardon the pun. It's very much a live issue in contemporary philosophy and as you'd guess, there is more than one camp.

Very roughly, we can carve the territory up this way:

Presentists say that only the present is real. The future does not exist, and will only come into being after the present has slipped away. One reason why some people defend presentism is that they believe it does more justice to our sense of the passage of time. On this view, things "become."

Eternalists think that all events are equally real. There is no special moment that counts as "the present." Rather, for any given moment, there are earlier and later moments. One reason (though not the only one) why some people defend eternalism is that it seems to fit better with the understanding of space and time that we get from the theory of relativity. Think of it this way. Suppose an event happens outside my window as I'm writing this—say, a car backfires. And suppose that on a planet far away in another galaxy, there's a certain event—an explosion, say—that happens, but as it works out no light signal, no matter how powerful, could get from here to that explosion, not could any other signal. To put it in the language of relativity, suppose the explosion on the other planet is outside the lightcone of the the backfire of the car. In that case, relativity says there's simply no answer to the question of which event happened first: backfire or explosion. In fact, if we say that only the present is real, we have no good way of saying what's part of the supposed present of any event.

That's a bit quick and dirty, but it may help give you the bigger picture. There's a big literature on this topic. One reader-friendly place to start is with Craig Callender's little book Introducing Time . Don't be put off by the fact that it has lots of cartoon pictures. It's very good. And you can even download it as an ebook.

A timely topic, if you'll pardon the pun. It's very much a live issue in contemporary philosophy and as you'd guess, there is more than one camp. Very roughly, we can carve the territory up this way: Presentists say that only the present is real. The future does not exist, and will only come into being after the present has slipped away. One reason why some people defend presentism is that they believe it does more justice to our sense of the passage of time. On this view, things "become." Eternalists think that all events are equally real. There is no special moment that counts as "the present." Rather, for any given moment, there are earlier and later moments. One reason (though not the only one) why some people defend eternalism is that it seems to fit better with the understanding of space and time that we get from the theory of relativity. Think of it this way. Suppose an event happens outside my window as I'm writing this—say, a car backfires. And suppose that on a planet far...

I've read that as we go faster time dilates and so time slows down. So my

I've read that as we go faster time dilates and so time slows down. So my question is that If suppose a person in a spacecraft accelerates to the speed of light. After sometime (in his prospective) he decides to decelerate finally to much much lower than the speed of light. Then during all of this how much time will have passed for everything outside? Will he be able to decelerate at all? I mean for an outside observer, who by some means, is able to see everything that is happening in the spaceship, will the person be frozen (in time) and therefore not able to push the button that decelerates the ship and ultimately travel infinitely in time and space? (again another assumption that the fuel does not run out). And (in the prospective of the space traveler) after pushing the button where will he be in time with respect to the observer? I hope I am able to convey my problem. Thanks in advance.

A good question. The nub of the matter is this: if something is moving literally at light speed, then the amount of time between two points along its trajectory, measured in its frame, is 0. So your hunch is right in one way: at light speed (along the edge of a light cone), time doesn't pass.

However, if we try to accelerate a massive body (for example, a spaceship) to light speed, we'll fail. Close enough for present purposes, the reason is that as the body accelerates, it takes on mass, and that mass approaches infinity as the speed approaches c. So your space traveler will never arrive at "frozen" time.

A good question. The nub of the matter is this: if something is moving literally at light speed, then the amount of time between two points along its trajectory, measured in its frame, is 0. So your hunch is right in one way: at light speed (along the edge of a light cone), time doesn't pass. However, if we try to accelerate a massive body (for example, a spaceship) to light speed, we'll fail. Close enough for present purposes, the reason is that as the body accelerates, it takes on mass, and that mass approaches infinity as the speed approaches c . So your space traveler will never arrive at "frozen" time.

What is time? My friend and I are having an argument about the nature of time.

What is time? My friend and I are having an argument about the nature of time. If I understand her position correctly my friend believes that time is simply an artefact of changes in the universe, or that change is itself synonymous with time. This seems to be a commonly accepted position, however I replied that if two spheres were moving in space in parallel to one another but one sphere was moving faster than the other then that would mean that that sphere would traverse a greater distance in less time but with an equal degree of change in the universe. Thus time and change are two separate things. I guess the physics of that claim are debatable but I suspect that it demonstrates something true in a way that is a priori irrespective of the physics because even if there were a physical difference in the amount of forces which change it would be hard to see how those imperceptible forces would contribute to a difference in how we perceive the time effect of those different objects. What do you...

Some good questions.

One view is that if there is change, there is time. However, it doesn't follow from that that time has a "metric" -- that there is an answer to questions of the sort "how much time?" If all that existed were two solid spheres in relative motion, then someone might say that there's no answer to the question "What's the relative velocity?" (hence how much time has passed between varying degrees of separation) even though there is change going on, and hence there's time.

More generally we can at least imagine a universe where time _order_ among events -- what happens before, after or simultaneously with what -- is definite, but without there being any correct answers to questions such as "How much time passed between event x and event y?" Insofar as that's right, it provides a way around your objection.

Whether time requires change is yet another question. Not everyone agrees that it does. Sidney Shoemaker, in a paper from some years ago, argues that there could be definite periods of time with no change. The reference is: “Time Without Change,” Journal of Philosophy 66 (1969), pp. 363-381. You might also look at the article on time in the Stanford Encyclopedia of Philosophy:

http://plato.stanford.edu/entries/time/

Good luck with your explorations!

Some good questions. One view is that if there is change, there is time. However, it doesn't follow from that that time has a "metric" -- that there is an answer to questions of the sort "how much time?" If all that existed were two solid spheres in relative motion, then someone might say that there's no answer to the question "What's the relative velocity?" (hence how much time has passed between varying degrees of separation) even though there is change going on, and hence there's time. More generally we can at least imagine a universe where time _order_ among events -- what happens before, after or simultaneously with what -- is definite, but without there being any correct answers to questions such as "How much time passed between event x and event y?" Insofar as that's right, it provides a way around your objection. Whether time requires change is yet another question. Not everyone agrees that it does. Sidney Shoemaker, in a paper from some years ago, argues that there could be...

I was recently speaking with someone who had an argument about whether time

I was recently speaking with someone who had an argument about whether time exists or not. Time Dilation is often put forward as proof that time exists and that it is not merely a figment of imagination of mankind. But this person argued that by believing this, we are making a self-contained assumption about time. He argued that time is actually just the measurement of change that occurs in an object relative to constant natural phenomena. For instance, atomic clocks measure the microwave emissions of changing electrons, and older clocks measure the degree of the earth's rotation. He suggests that it is a huge and erroneous jump to say that these things measure anything other than what is stated i.e. emissions of changing electrons or the degree of the earth's rotation, and that to say that they measure a force or external entity that is time is simply illogical. And I would be inclined to agree. I watched a Stephen Hawking program where he discussed the possibility of time travel. He spoke about the...

As you may sense yourself, part of the problem is to decide just what question we're asking and what would count as an answer.

Start with space. One way of claiming that space is real is to say that it's a thing -- a "substance", as the jargon of philosophy would have it. (In this sense, a substance is something that _has_ properties or qualities and stands in various relations, but isn't a property or a relation. An atom is a substance in this sense; so is an animal.)

The "substantival" view of space-time has its adherents, but also has its detractors. There are many philosophers who don't think space is real if that would imply that it's some kind of thing. Typically such philosophers would say that instead of talking about space as such, we should talk about spatial relations. Things can be between other things, or beside them or above the, or longer than them or a certain distance away from them. What's real, on this view, are the things that stand in the various relations; space isn't something above and beyond all those relational facts.

Obviously we could think of time in much the same way: time isn't a thing or a substance; rather, events (happening, occurrences) have relations such as earlier than, simultaneous with, ten minutes later than... among them. On this sort of view, that's all there is to the notion of time; it's not a thing, let alone a thing that influences more ordinary objects.

Once again, there are plenty of philosophers who prefer this view, and if talk of the reality of time is talk of there being some special _thing_, then lots of non-crazy people don't believe that time is real in that sense. What's real are things of more ordinary sorts (including the not-so-ordinary things of physics) and events of various kinds. Some of the facts about these things are facts about distance, relative location, before-and-after, etc. This ia an attractive thought because it takes away a lot of mystery: if time is a thing, what sort of thing could it possibly be?

Whatever we make of this view, it doesn't exhaust the issue. We can ask: are spatial and temporal relations objective? Or are they conventional? Is it an objective fact that the clock we use to measure time ticks regularly? Or do we _decide_ for various reasons to treat one kind of time-keeping device as the standard rather than another without there being any objective fact about whether it really ticks regularly? This isn't an easy issue, and there's lots of disagreement about it among philosophers.What we see, however, is that even if space and time are matters of relations, people can differ over the objectivity and hence, in an important sense, the _reality_ of the relations.

Your questions about relativity are good ones and they don't have easy answers. As it's often presented, relativity seems to fit best with something like a substance view of space-time. I say "space-time" because in relativity space and time are mathematically related in particularly intimate ways. According to general relativity, space-time doesn't obey the laws of Euclidean geometry, and in fact it departs from the Euclidean to varying degrees and in various ways depending on the background configuration of matter. In fact, on one common way of putting it, space-time tells matter how to move, matter tells space-time how to curve. On this view, space-time acts on matter and matter acts on space-time. If that's right, space-time seems pretty real. (Keep in mind: in relativity, time isn't something distinct from space. What's at issue are questions about space-time.)

The issues here are complicated, but there's no question of a mere leap in logic. General relativity is a complex and subtle account of spatio-temporal matters. On its face, it seems to posit something akin to the electromagnetic field, except it's a field that acts on all matter. Whether that's the best way to look at it is not an easy question, but what seems clear is that the view isn't the result of any simple mistake in reasoning. It's a matter of how best to make sense of a complicated web of theory and fact.

As you may sense yourself, part of the problem is to decide just what question we're asking and what would count as an answer. Start with space. One way of claiming that space is real is to say that it's a thing -- a "substance", as the jargon of philosophy would have it. (In this sense, a substance is something that _has_ properties or qualities and stands in various relations, but isn't a property or a relation. An atom is a substance in this sense; so is an animal.) The "substantival" view of space-time has its adherents, but also has its detractors. There are many philosophers who don't think space is real if that would imply that it's some kind of thing. Typically such philosophers would say that instead of talking about space as such, we should talk about spatial relations. Things can be between other things, or beside them or above the, or longer than them or a certain distance away from them. What's real, on this view, are the things that stand in the various relations; space isn't...

If time is infinite does this give us any hope for life after death? After all

If time is infinite does this give us any hope for life after death? After all if time is infinite, it is inevitable that all the cells in my body (my DNA etc) will be reconstructed in some far off day and age.

I'm not quite ready to go along with my colleague's answer, but my answer isn't any more hopeful.

If time has the structure of the real line (as we usually think) then even if it's infinite, every moment is only a finite time away from now. (Compare: every real number is only a finite distance from 0.)

But even if time is infinite in the way the real number line is, it doesn't follow that there will be a duplicate of you somewhere off in the future. To get that conclusion woud take a lot of extra and optional premises. More important, even if there will be a duplicate of you someday, there's no good reason to think it would be you, nor is there any good reason to think that you could look forward to its experiences. (These two aren't quite the same issue, as it turns out.)

Clearly there's a lot in the background here. If you're interested in more reading on the core problem, i.e., the problem personal identity, you might have a look at Martin and Barresi's anthology, called Personal Identity.

I'm not quite ready to go along with my colleague's answer, but my answer isn't any more hopeful. If time has the structure of the real line (as we usually think) then even if it's infinite, every moment is only a finite time away from now. (Compare: every real number is only a finite distance from 0.) But even if time is infinite in the way the real number line is, it doesn't follow that there will be a duplicate of you somewhere off in the future. To get that conclusion woud take a lot of extra and optional premises. More important, even if there will be a duplicate of you someday, there's no good reason to think it would be you , nor is there any good reason to think that you could look forward to its experiences. (These two aren't quite the same issue, as it turns out.) Clearly there's a lot in the background here. If you're interested in more reading on the core problem, i.e., the problem personal identity, you might have a look at Martin and Barresi's anthology, called Personal...

Is time an independent physical dimension or a human construct designed to

Is time an independent physical dimension or a human construct designed to compare events to each other ? If it is a physical entity why can we move only in one direction and at an inexorable pace? Is it theoretically possible for a time machine (Hot Tub or any other sort) could exist?

I agree with everything that Allen writes in his last comment. Some time travel scenarios are ruled out a priori : these are the inconsistent ones, and there may be others, for all I know. Are the consistent ones ruled out by anything? I can't see that they are, as the only reason I am clear about for thinking time travel is possible is the grandfather paradox. But it may only rule out the inconsistent cases. So I am in agreement with Allen here too, and in the dark as to whether anything in physics allows or rules out non-contradictory time travel. Time is a dimension, and dimensions are things that allow you to scale. A direction in the structure of the dimension itself seems a slightly incoherent idea to me, as opposed to the direction of the thing moving in the dimension, e.g. a place moving through colour space, such as the sky going from blue to red, or a bullet moving from there to here.

Just a footnote on Jonathan's reply on the matter of direction. Length is a measure of a property of things, and it has a natural 'direction' from shorter to longer. As Jonathan suggests, it wouldn't make any sense to say that the difference between one direction and another on the length scale is just a matter of convention. But it may be that position coordinates are closer to your worry. We can assign position coordinate so that heading north from my desk gives us bigger numbers. Or we could do it the other way: going north fives us smaller coordinates. And in this case, we'd say that the choice really is just convention. Nature doesn't favor one direction in space over another. But time seems to be different. There seems to be a real difference between the direction we label with increasing numbers and the opposite direction. As it turns out, physicists and philosophers have written a great deal about this asymmetry. As it also turns out, there isn't a consensus about the best way to think of it. Cups...

Just to continue the conversation - Jonathan and I agree that time is a dimension and not a force. It's just that this still leaves room for interesting questions about the relationship between thermodynamics and fundamental physical laws. We don't agree, it seems, about time travel, but we may agree for "all practical purposes." I'm quite satisfied with David Lewis's treatment of the grandparent paradox. Lewis agrees: you can't travel back in time and kill grandpa or grandma. Any such story is inconsistent, and inconsistent stories are guaranteed to be false. In fact, loosely put, you can't go back in time period unless it's actually a part of the world's history that it happened. Unless it's "already" a fact about the world that an adult Allen Stairs was wondering around the streets of his boyhood home in the 50s and 60s, then we are guaranteed that I will never do any such thing. Lewis's point was that in spite of this, we can tell consistent time travel stories. They just require very...

Kurt Gödel's incompleteness theorems represent one of the foremost achievements

Kurt Gödel's incompleteness theorems represent one of the foremost achievements in mathematical logic's proud history. AskPhilosopher's very own panelist Peter Smith obviously is greatly intrigued by these theorems. Suppose out of jealously -- though I doubt he would succumb to such a vice! -- he decided to build a time machine. Imagine, moreover, that he went to a time before Gödel had proven the theorems and gave the great logician, say, the idea of Gödel numbering, the key to proving the incompleteness theorems. My question thus is as follows: Who would deserve credit for proving the incompleteness theorems? Gödel seems to have gotten the idea from Peter; Peter seems to have gotten the idea from Gödel. Is it possible that neither would deserve credit?

What a lovely question!

The first thing to ask is whether the story is internally consistent -- unlike a story in which John kills his grandfather before Grandfather fathers John's mother. That appears not to be a problem here; there's no obvious hint of an event having to have happened and not happened. Instead, we simply have surprising set of internally consistent occurrences.

On, then, to responsibility. Since we have a causal loop here, there's no clear way to say which of Peter or Kurt is causally responsible for the theorem's having come to be stated and proved. If pressed, we might say that each gets equal causal credit,m though your mileage might vary on the apportionment. Indeed, neither is the originator or initiator; the most that can be said is that both were crucial parts of the process through which the theorem came to be.

What of intellectual credit?

Well, Peter didn't think the ideas up on his own. He learned them by reading about them, having been taught them, working through the proof and what not. So he doesn't get any intellectual credit. But as this story goes, Gödel didn't think the thing up either. (To make the story as perplexing as possible, we should assume that Peter simply gave the whole shebang to Kurt, rather than just giving him a crucial bit.) And so Gödel gets no intellectual credit either. Somehow, the world just burped forth Gödel's theorem, praise be to the Lords of Time!

Wonder if it really happened that way?

What a lovely question! The first thing to ask is whether the story is internally consistent -- unlike a story in which John kills his grandfather before Grandfather fathers John's mother. That appears not to be a problem here; there's no obvious hint of an event having to have happened and not happened. Instead, we simply have surprising set of internally consistent occurrences. On, then, to responsibility. Since we have a causal loop here, there's no clear way to say which of Peter or Kurt is causally responsible for the theorem's having come to be stated and proved. If pressed, we might say that each gets equal causal credit,m though your mileage might vary on the apportionment. Indeed, neither is the originator or initiator; the most that can be said is that both were crucial parts of the process through which the theorem came to be. What of intellectual credit? Well, Peter didn't think the ideas up on his own. He learned them by reading about them, having been taught them,...

Are there any good philosophical reasons for thinking that time travel is

Are there any good philosophical reasons for thinking that time travel is possible?

Yes and no. Let me explain.

Some people think that is flat-out impossible. They appeal, for example, to puzzles like the Grandfather Paradox: if time travel were possible, the argument goes, I would be able to go back to 19xx and kill my Grandfather before he met my Grandmother. This would mean that I would never exist, and so the scenario requires both that I do and that I don't exist: contradiction. This is meant to show that time travel is impossible. Philosophers can help us sort through that sort of problem, and in fact some have. David Lewis's "The Paradoxes of Time Travel" (you can find it in his Philosophical Papers, volume II) is still a lucid and useful sorting of the issues. Lewis argues -- correctly, I think -- that the Grandfather Paradox doesn't show what it's meant to. Roughly, the idea is this: if I do someday travel back to 19xx gunning for Grandpa, then first, it's true even now in 2008 that there was a deranged philosopher lurking around in those days gunning for Earl Stairs. (You may dooubt that this is so; so do I. But do either of us know it beyond all doubt?) Second, however, we can say for sure that I failed. I didn't manage to kill Grandpa, because he did actually marry Grandma, and they did actually have a son who married my mother and became my father. Time travel stories have to be consistent, but consistent time travel stories are possible. So philosophers have offered reasons to think that time travel is possible in the broadest sense: it's not inherently incoherent -- it's logically possible or metaphysically possible.

If we take philosophers like Lewis to have done that part of the job correctly, however, there's another question that calls for a different sort of expertise: do the laws and conditions that hold in this universe allow for time travel? Roughly, is time travel physically possible? That's not something that philosophical reflection alone can tell us. General relativity, our best theory of space and time, apparently allows for "closed time-like curves," and following one of those would amount to time travelling. But "allows for" is pretty abstract. It doesn't follow that there's any way of arranging this that a human body could survive. And it also doesn't follow that the technology needed to generate those curves is within our capability. So on the most general question, yes: philosophers can give us reasons to think time travel is possible. But when it gets down to the nitty gritty, physicists and physiologists and engineers would have to weigh in.

(I've assumed, by the way, that you had travel into the past in mind. Ordinary "time travel" into the future is no problem; that's how we got from yesterday to today. And relativity straightforwardly allows for more interesting variants on future-oriented time travel. If you did it the right way, you could get from Times Square, Jan 1 2009 to Times Square, Jan 1 2010 in less than a year, by your reckoning. Hermann Bondi's Assumption and Myth in Physical Theory is still a friendly introduction to those ideas 40 years after it was written.)

Yes and no. Let me explain. Some people think that is flat-out impossible. They appeal, for example, to puzzles like the Grandfather Paradox: if time travel were possible, the argument goes, I would be able to go back to 19xx and kill my Grandfather before he met my Grandmother. This would mean that I would never exist, and so the scenario requires both that I do and that I don't exist: contradiction. This is meant to show that time travel is impossible. Philosophers can help us sort through that sort of problem, and in fact some have. David Lewis's "The Paradoxes of Time Travel" (you can find it in his Philosophical Papers , volume II) is still a lucid and useful sorting of the issues. Lewis argues -- correctly, I think -- that the Grandfather Paradox doesn't show what it's meant to. Roughly, the idea is this: if I do someday travel back to 19xx gunning for Grandpa, then first, it's true even now in 2008 that there was a deranged philosopher lurking around in those days gunning for Earl Stairs. ...

In class, our professor discussed the impossibility of time travel. He stated

In class, our professor discussed the impossibility of time travel. He stated that if in the future, machines are made to travel back into time, then we would be seeing people from the future right now. His argument ended there but would this be true? Is this a valid argument to disprove the possibility of time traveling in the future?

I hope your professor was just trying to provoke you, because it's a terrible argument. For one thing, it's not clear why he's so sure that we aren't already seeing people from the future, who've traveled back to this time zone, as it were, and are doing a good job of blending in. And in any case, suppose that in 3008, someone figures out how to travel backward in time. Why is it so obvious that they would come to this time?" Why not a later time? Or a time when there were no humans at all? If we add the plausible conjecture that the process would be expensive, dangerous and not altogether reliable, what basis would we have at all for speculating about the likelihood that someone would have shown up somewhere that we'd know about?

More importantly, if something is actual, it's certainly possible, but the converse doesn't follow. Even if time travel is possible, it doesn't follow that it will ever actually happen. The world is and always will be pregnant with unrealized possibilities. Perhaps this is one of them.

And then there's the question "possible in what sense?" The equations of general relativity allow solutions that include so-called "closed timelike curves." Anything that followed such a curve could fairly be said to undergo time travel. But from the fact that this is physically possible, it doesn't follow at all that it's a practical possibility. The laws of physics allow many things that we'll never be in a position to do.

Though your professor doesn't seem to have this in mind, some people have argued that time travel is inherently paradoxical and therefore that the whole notion is incoherent. (For example: the idea of time travel is supposed to sanction the possibility that I could kill my own grandfather before he ever met my grandmother, which would mean that I could cause myself never to have existed.) I think this is confused, and I can do no better than recommend that you get a copy of David Lewis's paper "The Paradoxes of Time Travel" (American Philosophical Quarterly 13: 145-52. It's also reprinted in his Philosophical Papers Volume II. Oxford: Oxford University Press.)

I hope your professor was just trying to provoke you, because it's a terrible argument. For one thing, it's not clear why he's so sure that we aren't already seeing people from the future, who've traveled back to this time zone, as it were, and are doing a good job of blending in. And in any case, suppose that in 3008, someone figures out how to travel backward in time. Why is it so obvious that they would come to this time?" Why not a later time? Or a time when there were no humans at all? If we add the plausible conjecture that the process would be expensive, dangerous and not altogether reliable, what basis would we have at all for speculating about the likelihood that someone would have shown up somewhere that we'd know about? More importantly, if something is actual , it's certainly possible, but the converse doesn't follow. Even if time travel is possible, it doesn't follow that it will ever actually happen. The world is and always will be pregnant with unrealized possibilities....

Great site. How does our approach to knowledge about the past differ from our

Great site. How does our approach to knowledge about the past differ from our approach to knowledge about the future?

Others may have things to add, but one obvious way is that many of our beliefs about the past are caused by things that happened in the past and produced traces, either directly or indirectly, in our brains. But on the usual view about how the universe is wired up, our beliefs about the future aren't caused by future events.

This doesn't make knowledge claims about the past uniformly more secure than knowledge claims about the future. Some facts about the past may be well nigh inaccessible; their traces may be faint or non-existent, and there may be no good general grounds for inferring. (For example: I'd guess that there's almost no hope that anyone will ever know exactly how many people were on the swath of ground now marked out by the University of Maryland campus at noon on April 3, 1808. But -- skeptical worries aside -- we can reasonably claim to know that the earth will rotate on its axis over the next 24 hours.

Still, knowledge of the past has a certain priority. Our knowledge that the earth will rotate on its axis over the next 24 hours is based on things we know about the past and generalizations that this knowledge supports. Something like this is true in general: knowledge of future events is grounded in knowledge of the past, but not vice-versa.

Others may have things to add, but one obvious way is that many of our beliefs about the past are caused by things that happened in the past and produced traces, either directly or indirectly, in our brains. But on the usual view about how the universe is wired up, our beliefs about the future aren't caused by future events. This doesn't make knowledge claims about the past uniformly more secure than knowledge claims about the future. Some facts about the past may be well nigh inaccessible; their traces may be faint or non-existent, and there may be no good general grounds for inferring. (For example: I'd guess that there's almost no hope that anyone will ever know exactly how many people were on the swath of ground now marked out by the University of Maryland campus at noon on April 3, 1808. But -- skeptical worries aside -- we can reasonably claim to know that the earth will rotate on its axis over the next 24 hours. Still, knowledge of the past has a certain priority. Our knowledge that the...

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