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The following program features simulated voices generated for educational and philosophical exploration.
Leonard Jones
Good afternoon. I'm Leonard Jones.
Jessica Moss
And I'm Jessica Moss. Welcome to Simulectics Radio.
Leonard Jones
Today we're examining one of metaphysics' most fundamental questions: what exists temporally? Does only the present moment exist, or are past and future equally real? This debate between presentism and eternalism concerns the very structure of temporal reality.
Jessica Moss
The stakes here are profound. How we answer shapes our understanding of change, persistence, free will, and even our relationship to our own past and future selves. It connects to physics, personal identity, and the nature of becoming.
Leonard Jones
Our guest is Dr. Dean Zimmerman, Professor of Philosophy at Rutgers University, whose work on the metaphysics of time, persistence, and material objects has been influential in contemporary analytic metaphysics. Dr. Zimmerman, welcome.
Dr. Dean Zimmerman
Thank you. These questions about the reality of past and future touch on some of the deepest issues in metaphysics.
Jessica Moss
Let's start with definitions. What exactly is presentism claiming?
Dr. Dean Zimmerman
Presentism is the view that only present objects and present times exist. The past did exist, and the future will exist, but they don't exist now. When we talk about Caesar crossing the Rubicon, we're not referring to something that exists—that event is gone. Only what exists presently is real. It's a very natural view that aligns with how we experience time as flowing.
Leonard Jones
And eternalism is the alternative?
Dr. Dean Zimmerman
Eternalism, also called the block universe view, says that past, present, and future all exist equally. All times are equally real. From this perspective, the flow of time is an illusion. The universe is a four-dimensional block where all events—past, present, and future—simply exist at their respective temporal locations. Caesar crossing the Rubicon exists just as much as what's happening right now.
Jessica Moss
That seems deeply counterintuitive. Our experience suggests the present is special, that the past is gone and the future hasn't happened yet.
Dr. Dean Zimmerman
That's one of presentism's main attractions—it captures our phenomenology of time. We seem to experience genuine passage, a moving present that brings new things into existence and consigns others to non-existence. Eternalism requires explaining away this appearance as merely perspectival, like how objects appear smaller at a distance even though they're not really smaller.
Leonard Jones
Let me be precise about this. What's the difference between something not existing now versus something not existing at all? When presentists say the past doesn't exist, what exactly are they denying?
Dr. Dean Zimmerman
That's a crucial question. Presentists distinguish between tensed and tenseless existence. Caesar doesn't exist simpliciter—there's no Caesar in the universe's current inventory of existing things. But it was the case that Caesar exists—we can make true past-tensed claims about him. The eternalist says Caesar exists tenselessly at his temporal location, the way things exist at spatial locations. The presentist denies this—only what exists in the present tense genuinely exists.
Jessica Moss
How does special relativity bear on this debate? I've heard it poses problems for presentism.
Dr. Dean Zimmerman
This is one of the strongest arguments for eternalism. Special relativity tells us that simultaneity is relative to reference frames. What counts as 'now' depends on your state of motion. Two events that are simultaneous in one frame aren't simultaneous in another. If presentism is true and only the present exists, then what exists depends on your reference frame. That seems to make existence frame-relative, which is very strange.
Leonard Jones
Can the presentist respond that there's a privileged reference frame that determines absolute simultaneity, even if relativity doesn't tell us which one it is?
Dr. Dean Zimmerman
That's one strategy. You could say relativity is empirically adequate but not metaphysically complete—there's a fact about absolute simultaneity that goes beyond what relativity describes. The problem is this feels ad hoc. We're positing metaphysical structure that makes no empirical difference. And it contradicts the spirit of relativity, which suggests there's no privileged frame.
Jessica Moss
What about the growing block view? Doesn't that offer a middle position?
Dr. Dean Zimmerman
Yes. The growing block view says the past and present exist, but the future doesn't. The universe is a four-dimensional block that's constantly growing as new present moments come into existence. This captures the asymmetry between past and future—the past is fixed and real, the future is genuinely open—while avoiding some of presentism's problems.
Leonard Jones
But doesn't the growing block view face similar problems with relativity? What determines where the edge of the block is?
Dr. Dean Zimmerman
Exactly. You still need absolute simultaneity to determine which events are being added to the block at any given moment. The growing block view doesn't escape the tension with relativity—it shares presentism's need for a privileged foliation of spacetime.
Jessica Moss
Let's talk about truthmakers. How do presentists account for truths about the past if past events don't exist?
Dr. Dean Zimmerman
This is a serious challenge. If Caesar crossing the Rubicon doesn't exist, what makes it true that Caesar crossed the Rubicon? The eternalist has an easy answer—the truth is made true by Caesar's past action, which exists at its temporal location. The presentist needs to find truthmakers in the present for past-tensed truths.
Leonard Jones
What options do presentists have?
Dr. Dean Zimmerman
One option is to say presently existing objects have irreducible past-directed properties—properties like having-crossed-the-Rubicon. These presently instantiated properties ground past-tensed truths. Another option is to accept that some truths don't need truthmakers—past-tensed truths are primitive and brute. A third option appeals to God's memory, if you're a theist—past truths are grounded in God's present knowledge of the past.
Jessica Moss
The first option seems to smuggle in the past through these special properties. Don't those properties essentially encode the entire past?
Dr. Dean Zimmerman
That's a fair criticism. If objects carry all this information about the past, you might wonder what metaphysical work is being done by denying the past's existence. The eternalist can argue we're just relabeling—instead of past events existing, we have present objects with infinitely many past-oriented properties. It's not clear that's really more ontologically parsimonious.
Leonard Jones
What about the problem of persistence? How do objects persist through time on these different views?
Dr. Dean Zimmerman
Eternalists typically embrace four-dimensionalism or perdurantism—objects are extended in time the way they're extended in space. You're a four-dimensional spacetime worm with different temporal parts. The you of yesterday is a different temporal part than the you of today. Presentists typically favor three-dimensionalism or endurantism—objects are wholly present at each moment they exist. You're the same three-dimensional entity existing at different times.
Jessica Moss
How does three-dimensionalism work if only the present exists? Doesn't the object need to exist at past times to persist through them?
Dr. Dean Zimmerman
The presentist says persistence is a matter of the same object existing at successive presents. The object doesn't exist at past times now—it did exist at them when they were present. Persistence is primitive succession of existence, not existing at multiple times simultaneously. The eternalist pictures all your temporal parts existing together in the block universe, but the presentist sees you existing at one time after another.
Leonard Jones
There's a question about change here. How do these views understand what it is for something to change?
Dr. Dean Zimmerman
For the eternalist, change is having different properties at different temporal locations—having different temporal parts with different properties. A leaf changes from green to red by having a green temporal part in summer and a red temporal part in fall. The presentist says change is the same object having different properties at successive times—the same leaf being green, then later being red.
Jessica Moss
The presentist view seems to capture our intuitive notion of change better. On eternalism, is anything really changing if all the temporal parts just exist statically in the block?
Dr. Dean Zimmerman
That's a powerful intuition. Eternalists often respond that change just is variation across temporal dimension, analogous to variation across space. A road can be bumpy without the road itself changing—different parts have different properties. Similarly, a four-dimensional object can change without any becoming—different temporal parts have different properties. But you're right that this seems to miss something about our ordinary notion of change.
Leonard Jones
What about the open future? Does presentism leave room for genuine indeterminacy about what will happen?
Dr. Dean Zimmerman
Presentism is naturally compatible with an open future. If the future doesn't exist, there may be no fact of the matter about what will happen—the future is genuinely undetermined. This connects to libertarian free will. If my future choices don't yet exist, perhaps I'm genuinely free to make them in multiple ways.
Jessica Moss
Can eternalists accommodate an open future?
Dr. Dean Zimmerman
It's harder. If future events exist in the block universe, it seems they're already determinate. Some eternalists embrace this—they accept that the future is fixed even if we don't know it. Others try to combine eternalism with indeterminacy by saying there are multiple possible futures that all exist, but it's indeterminate which one is actual. That's a complicated view with its own problems.
Leonard Jones
Let me ask about the metaphysics of temporal becoming. What is the moving present on presentism?
Dr. Dean Zimmerman
This is surprisingly hard to articulate clearly. We want to say the present moment moves or changes, but change requires existing at multiple times. If only the present exists, there's no standpoint from which to observe the present changing. One approach is to say becoming is primitive and irreducible—it's just a basic feature of reality that new things come into existence.
Jessica Moss
McTaggart argued that temporal becoming is contradictory. What's your view of that argument?
Dr. Dean Zimmerman
McTaggart distinguished the A-series—past, present, future—from the B-series—earlier than, later than. He argued the A-series is essential to time but contradictory, because events must have incompatible properties like being past, present, and future. The standard presentist response is that these aren't incompatible—an event can be future, then present, then past at different times. But this requires temporal change in an event's temporal properties, which McTaggart argued leads to regress.
Leonard Jones
How do you avoid the regress?
Dr. Dean Zimmerman
You can deny that temporal properties themselves change—rather, different times have the fundamental property of being present. Or you can embrace the regress but argue it's not vicious. These are active areas of debate. McTaggart's argument is ingenious, but I don't think it's decisive against the A-theory.
Jessica Moss
What about retrocausation and time travel? Do these possibilities favor one view over another?
Dr. Dean Zimmerman
Eternalism makes time travel conceptually easier. If all times exist, you can just move from one temporal location to another. Presentism faces the problem that past times don't exist anymore—how can you travel to something that isn't there? Though presentists can perhaps say time travel involves causing past events that did exist when they were present.
Leonard Jones
There's a question about the relationship between metaphysics and physics here. Should metaphysics defer to our best physics, or can it legitimately posit structure beyond what physics describes?
Dr. Dean Zimmerman
That's a methodological question that divides philosophers. Some think metaphysics should be continuous with science—we shouldn't posit undetectable metaphysical structure. Others think metaphysics addresses questions science doesn't—questions about what fundamentally exists, not just what's empirically useful to posit. I lean toward the latter, but I acknowledge the force of arguments for naturalistic constraints on metaphysics.
Jessica Moss
How does this debate connect to other areas of philosophy—ethics, philosophy of mind, epistemology?
Dr. Dean Zimmerman
It touches many areas. In ethics, if the future doesn't exist, perhaps we have different obligations toward it than toward the present. In philosophy of mind, our experience of temporal flow might reveal something about time's nature. In epistemology, how we know about the past depends on whether past events exist to be known about. The metaphysics of time is deeply interconnected with other philosophical questions.
Leonard Jones
Where does the debate stand currently? Is there emerging consensus?
Dr. Dean Zimmerman
Eternalism has become somewhat dominant, partly due to relativity and partly due to contemporary metaphysics' physicalist leanings. But presentism has sophisticated defenders who've developed responses to the main objections. The growing block view has seen renewed interest. I'd say the debate is very much alive, with none of the positions decisively refuted.
Jessica Moss
What's your own view? Are you a presentist or eternalist?
Dr. Dean Zimmerman
I'm sympathetic to presentism and have defended it against some objections, though I wouldn't say I'm fully committed. The phenomenology of temporal passage and the intuitive understanding of change pull me toward presentism. But the problems with relativity are serious, and I'm not entirely satisfied with available solutions. It's one of those cases where philosophical reflection leaves me in genuine uncertainty.
Leonard Jones
That's philosophically honest. The deepest questions often resist definitive resolution.
Jessica Moss
Dr. Zimmerman, you've given us a rigorous examination of temporal ontology and its implications. Thank you.
Dr. Dean Zimmerman
Thank you. These questions about what exists across time are as difficult as they are fundamental.
Leonard Jones
That's our program. Until tomorrow, consider whether the past still exists or has slipped into non-being.
Jessica Moss
And whether the future awaits you or doesn't yet exist to be awaited. Good afternoon.