In his Confessions, St. Augustine remarks about time that, “. . I know what it is if no one asks; but if anyone does, then I cannot explain it.”
It may seem easy to explain the present: it’s what’s happening, what’s real, now.
But are the past and future just as “real” as the present? And what does “real” mean in this context?
Here we’ll consider the three most popular positions on the reality of the past and future. Our discussion introduces the philosophy of space and time, of which the philosophy of time is one part. This area of philosophy—at the intersection of metaphysics and the philosophy of physics—helps us understand whether the past and future are real or not.
1. Presentism, Eternalism, and the Growing Block Theory
The present, as compared to the past and future, is unique in our experiences: we experience present events right when they happen.
Past events can only be accessed through memory, when other people share their memories, or by reviewing photos, writings, and similar objects.
We don’t currently experience future events, though we may experience them in the future; the best we can do is make predictions about what will be.
So the present moment is special in that, as we eat our lunch, we experience that meal directly. Past breakfasts and future dinners are not directly available to us.
Considering the special experience of the present can lead us to one of the following theories about the reality of the past, present, and future:
- Presentism: presentists argue that only the present is real and that the past and future are, in some sense, not real.
- Eternalism: eternalists hold that the past, present, and future are all equally real.
- Growing block theory: growing block theorists treat the past and present as real but the future as unreal.
These views present different pictures about the relationship between our experience and what is real. Presentists take our special access to the present to give us good reason to believe it is uniquely real. Eternalists and growing block theorists believe that our experiences of the present give us no more reason to think that the past (and future) is unreal than my experience of being in Georgia gives me reason to think that distant Madagascar doesn’t exist.
2. Relativity and Eternalism
While eternalists are accused of not explaining our special experience of the present moment adequately, its alternatives struggle to accommodate modern physics, particularly the theory of relativity.
Einstein’s theory of relativity unifies three-dimensional space and one-dimensional time into a single, four-dimensional spacetime. The four-dimensional “distance” between any two events, called the “spacetime interval,” is an objective fact, but observers moving at different speeds may disagree about how much time has elapsed between these events. This phenomenon, where observers traveling at different speeds may disagree with each other about which events happen at the same time, is called the “relativity of simultaneity.”
The relativity of simultaneity poses a problem for presentists and growing block theorists in scenarios like the following:
Bob sees a balloon outside his house pop at the same time that he sees the clock in his living room strike four. In Bob’s reference frame, these two events—the balloon popping and his clock striking four—happen at the same time. A second later, Bob sees a child crying in response to the balloon pop.
But an astronaut in her spaceship flying close to the speed of light over Bob’s house looks down and sees this:
Bob’s clock strikes four at the same time that she sees the child holding a popped balloon begin to cry. In her reference frame, the child crying and Bob’s clock striking four happen at the same time.
The puzzle for the presentist and growing block theorist is this:
From Bob’s perspective, when his clock strikes four, the balloon popping is in the present while the child’s crying is in the future. From the astronaut’s perspective, when Bob’s clock strikes four, the balloon popping is in the past, and the child’s crying is in the present.
Both the presentist and growing block theorist tell us that the future is unreal, but are they talking about Bob’s future or the astronaut’s? The theory of relativity doesn’t allow for special, privileged reference frames that tell us what’s real or what isn’t, so presentists and growing block theorists must explain how their theories accommodate relativistic phenomena.
3. Presentism and Language
We might understand the terms “real” and “unreal” in two different ways.
First, if by “real event” we mean one that’s actually happening right now. Someone asking “Is it really raining outside?” uses this tensed definition of “real”. On the tensed reading of “real”, the presentist position seems obvious and the alternative positions nonsensical—of course only present events exist right now! That’s what “now” means!
But there is also a tenseless definition of “real”, one that someone might use when, after seeing the musical Hamilton, they ask their friend, “Is Burr really Hamilton’s killer?” The questioner is not asking whether Burr is killing Hamilton right now but whether Burr’s killing Hamilton is an invention of Hamilton’s author or an actual, historical event.
When we use a tenseless definition to understand presentism and its alternatives, the presentist owes us reason to think that our special access to the present gives us good reason to believe that the past and future are unreal like Narnia, Hogwarts, or any other fictional setting.
Because presentism and its alternatives seem so different depending on whether one adopts a tensed or tenseless reading of these positions, some philosophers have argued that the debate over the reality of the past and future is merely semantic.
Contemplating the reality of the past and future can leave us feeling as confused as St. Augustine about the nature of time. But by taking time to consider our experience of time, our best available physical theories, and clearer definitions of the words we use, we can better understand it.
 Confessions 11.17.
 Before launching into this examination, we should address one question first: why the philosophy of space and time? Why not treat these concepts separately? After all, we don’t have a subfield of philosophy called philosophy of physics and aesthetics: we keep the two fields separate!
One answer is that many of our best physical theories treat space and time in a similar (if not identical) way – both space and time are dimensions that describe our physical world and provide us with a way to situate both ourselves and the events we care about. As a result, there are some puzzles in the philosophy of space that have analogues in the philosophy of time, and vice-versa.
But one of the best reasons to consider space and time together in one philosophical subfield comes from relativistic physics, which treats space and time as a single unified whole, spacetime, instead of as separate concepts. We’ll cover some considerations from relativity later in this entry, but see Sklar (1977) for more on the philosophical ramifications of relativity.
 There are, of course, other possible positions that we can take on the reality of the past, present, and future than those I list below: for instance, we could believe that only the past is real, or that only the future is real, or that only the past and future are real, or that only the present and future are real. However, to my knowledge, no philosophers have defended these theories as almost all philosophers of time seem to agree that at least the present moment is real and that the past is more likely to be real than the future.
I say “almost all” because there are some philosophers, following in the tradition of McTaggart (1908), who argue that all of time (past, present, and future) is unreal. While interesting, this argument is sufficiently complex to warrant its own separate entry, so I will restrict my attention to presentism, eternalism, and the growing block model for this entry and refer those interested in learning more about McTaggart and his argument to section 3 of McDaniel (2020).
 See, for instance, Lucas (1989), who writes that eternalism “gives a deeply inadequate view of time” because, among other things, it fails to “account for…the pre-eminence of the present…” (8).
 This style of argument was first presented by Rietdijk (1966) and Putnam (1967). My co-author Michael Silberstein and I presented an updated version of the argument in 2010.
 There are, of course, other options here aside from bringing presentism and the theory of relativity into harmony: some might reject relativistic physics as an adequate physical theory, and some might be willing to say that agents traveling at different speeds just live in different realities. In some ways, this second option isn’t too different from Stein’s (1991) “point presentism,” which restricts the notion of “the present” to things happening both right now and right here.
 Notably Dorato (2006) and Savitt (2006).
Peterson, Daniel and Michael Silberstein. (2010). “Relativity of Simultaneity and Eternalism: In Defense of the Block Universe.” In V. Petkov (ed) Space, Time, and Spacetime: Physical and Philosophical Implications of Minkowski’s Unification of Space and Time. New York, NY: Springer Press.
Philosophy of Space and Time: What is Space? by Dan Peterson
Time Travel by Taylor W. Cyr
Quantum Mechanics and Philosophy I: The Superposition of Paths by Thomas Metcalf
About the Author
Dan Peterson is an adjunct professor of philosophy at Morehouse College. He received his Ph.D. from the University of Michigan and specializes in the philosophy of physics, philosophy of science, and formal epistemology. He has research and teaching interests in metaphysics, philosophy of religion, philosophy of education, and ethics. He is also the co-founder and executive director of Mind Bubble, an educational nonprofit in Atlanta that provides local students with free tutoring and educational workshops. DanielJamesPeterson.com