Why the Block Universe Says You're a Stack of Temporal Parts

Right now, on a galaxy 2.5 million light-years away, an event is taking place. Or it already happened. Or it has not happened yet. According to the same physics that runs the GPS in your pocket, which of those is true can depend on how fast you are walking down the street.

And that same physics asks a stranger question about you. Are you a single, whole self moving through time one moment at a time? Or are you something else, a shape stretched across time, a four-dimensional object whose now is only one thin slice of a far larger thing?

This is the physics of identity in a block universe. The idea, taken seriously by Albert Einstein, by Hermann Minkowski, by Hilary Putnam, and by a century of thinkers after them, is that the past, the present, and the future may all exist together. And that you may not be a traveler through time at all, but a worldline laid out across it, assembled from what philosophers call temporal parts.

This is the picture, the argument that forces it, and where it quietly runs out of answers.

The vanishing of the universal “now”

In June 1905, a twenty-six-year-old patent clerk in Bern named Albert Einstein submitted a paper to Annalen der Physik titled “On the Electrodynamics of Moving Bodies.” It founded special relativity. The argument rested on two postulates of disarming simplicity. First, the laws of physics are the same for every observer moving at constant velocity. Second, the speed of light in vacuum is the same for every such observer, regardless of the motion of the source or the observer.

That second postulate is the strange one. The speed of light is 299,792,458 meters per second, fixed now by definition. No matter how fast you chase a beam of light, it recedes from you at exactly that speed. You cannot gain on it.

Hold that fixed, and something has to give. What gives is time. Two observers in relative motion will disagree about the time order of two events separated in space. This is the relativity of simultaneity, and it dissolves the universal now that almost every person takes for granted. There is no fact of the matter about what is happening right now on Andromeda, because two people walking past each other on a sidewalk will assign different times to events there. The disagreement, computed from the Lorentz transformation, can run to days.

Three years later, in September 1908, the mathematician Hermann Minkowski showed that Einstein’s equations described not three-dimensional space evolving through time but a single four-dimensional manifold. He gave it the name we still use: spacetime. In his lecture, he said that space by itself and time by itself were doomed to fade away into mere shadows. Only their union would preserve an independent reality.

In this picture, an event is not something that happens. An event is a point at four coordinates, three of space and one of time, in the four-dimensional manifold. Your birth is one such point. This sentence is another. Your death, whenever it falls, is a third. All of them sit in the same structure, all equally real.

What we call a person, on this view, is not an object moving through time. It is a worldline, a continuous curve through spacetime from the coordinates of birth to the coordinates of death, with every moment of consciousness located somewhere along the curve. The worldline does not move. It is, in the same way a road across a landscape is.

This is the block universe. And once you accept it, an old question turns sharp. What, exactly, are you?

Endurantism and perdurantism

Philosophers have a precise way of asking this. They call it the problem of persistence. In virtue of what is an object that exists at one time the very same object that exists at another?

The first answer is called endurantism. To endure is to be wholly present at every moment at which you exist. On this view, the entire you, complete and undivided, is fully here right now, and was fully there yesterday, and will be fully there tomorrow. There are not parts of you parceled out across time. There is one whole thing, persisting, present in its entirety at each instant. This is the view almost everyone holds the moment before they think about it.

The second answer is perdurantism. To perdure is to persist by having different temporal parts at different times. On this view, you are not wholly present at any single moment. What is present right now is only a part of you, a temporal part, a momentary slice, in the same way that only part of a road is present at any one milepost.

Philosophers call this worm theory, because a persisting object stretches through time the way an earthworm stretches through space. A long, thin four-dimensional thing, most of which is never where you happen to be looking. You at this moment are one segment of the worm. You yesterday are another. The whole worm is the entire four-dimensional creature.

There is a sharper cousin worth naming. Stage theory, defended by Theodore Sider and the British philosopher Katherine Hawley, says the real you is the present slice itself, and that the slices at other times are not parts of you but counterparts. On the stage view, the you of this instant is a complete but fleeting object, and you yesterday is a different object that this one is descended from. Either way, worm or stage, the four-dimensional ontology is the same. Reality is the block, and persons are patterns within it, made of temporal parts.

Why anyone takes the picture seriously

Nobody abandons the comfortable, whole, enduring self on a whim. The leading argument comes from a deep and genuine puzzle that has nothing to do with relativity and everything to do with ordinary change. Philosophers call it the problem of temporary intrinsics, and it was pressed hardest by the American philosopher David Lewis in his 1976 essay “Survival and Identity.”

Suppose you sit, and then you stand. Sitting and standing are intrinsic properties, properties you have in yourself, not in relation to anything else. But here is the difficulty. If one and the same wholly-present thing is bent while sitting and straight while standing, then it has two incompatible intrinsic shapes. A single thing cannot be both bent and straight, full stop.

One option is to say shape is not really intrinsic after all, that being bent is secretly a relation to a time. But that feels wrong. Your posture seems to be a fact about you, not a relation you bear to a clock.

The perdurantist offers a clean escape. There is no single thing that is both bent and straight. There is a bent temporal part and a different straight temporal part, each with its own intrinsic shape, strung along your worldline. The contradiction dissolves because the bearer of bent and the bearer of straight are not the same object. They are different parts of one worm.

A second puzzle reinforces the first. Take a lump of clay. On Wednesday, you mould it into a statue. On the following Sunday, you crush it back into a shapeless lump. The lump existed for the whole week. The statue existed for five days. But if the statue and the lump are the very same object, how can one of them be older than the other? The same thing cannot have two different lifespans.

The perdurantist has a tidy answer. The statue and the lump are two four-dimensional objects that share their temporal parts during those five days and diverge outside them. The statue is, in effect, a temporal part of the longer-lived lump. Two worms overlap along part of their length. The puzzle of coincidence becomes unremarkable.

This way of thinking did not arrive overnight. Willard Van Orman Quine, in his 1948 essay “On What There Is,” helped make respectable the idea of treating objects as four-dimensional. John Jamieson Carswell Smart, in his 1949 paper “The River of Time,” became one of the earliest defenders of the tenseless, B-theoretic view of time. And in 2001, Theodore Sider published Four-Dimensionalism: An Ontology of Persistence and Time with Oxford University Press. It became the canonical modern defense. Sider’s thesis is stated without hedging. The material world is composed of temporal parts as well as spatial parts. Cut the world along space and you get objects side by side. Cut it along time and you get temporal parts, one after another.

Pushing the picture into the self

The puzzles above concerned statues and postures. Push the four-dimensional picture into the place where it bites hardest, not statues but selves, and the sharpest instrument is a thought experiment philosophers call fission.

Imagine that the two hemispheres of a person’s brain could each be transplanted into a separate body, and that each resulting person woke up with the original’s memories, character, intentions, the whole psychological fabric intact. Call them Lefty and Righty. Each is fully psychologically continuous with the original. Now ask: which one is the original person?

It cannot be both, because one thing cannot be numerically identical with two distinct things. Lefty and Righty are clearly two people. They will live different lives, see different rooms, never share a thought again. But there is no principled reason to say it is Lefty rather than Righty, or Righty rather than Lefty. The case is perfectly symmetric. So the original is neither, or the question has no good answer. Personal identity, the strict relation of being one and the same person, has broken in our hands.

It was the English philosopher Derek Parfit (1942 to 2017) who drew the most influential conclusion from cases like these. First in his 1971 paper “Personal Identity,” then in his landmark 1984 book Reasons and Persons, Parfit defended a reductionist view of the self. The facts about a person consist in more particular facts, facts about a brain, a body, and a long chain of interlocking psychological connections: memories linking to experiences, intentions linking to actions, beliefs and desires carried forward and gradually edited. There is, Parfit argued, no further deep fact, no separately existing ego, no indivisible self over and above that chain.

And then he drew the conclusion that gives this whole topic its unsettling edge. In fission, your ordinary concern for your own future can attach to someone who is not, strictly, you. From this Parfit concluded that strict numerical identity may not be what matters. What matters, he said, is psychological continuity and connectedness, a relation he labeled Relation R. The slogan that crystallized it has echoed through philosophy ever since: identity is not what matters.

In the four-dimensional picture, this lands with peculiar force. If you are a worm of temporal parts, then you is the name of a pattern, a thread of connections running along a worldline, and the deep unified self was a story the thread told about itself.

Voices on the other side

Not everyone accepts this thinning-out, and the resistance is serious.

One major alternative is animalism, defended by the philosopher Eric Olson in his 1997 book The Human Animal: Personal Identity Without Psychology. Animalism says you are, quite literally, a biological organism, a living human animal, and that your persistence is the persistence of that organism, not of a psychological pattern that could in principle be copied, split, or uploaded. On this view, fission puzzles lose much of their bite, because organisms do not ordinarily divide that way, and what you are is settled by biology, not by the philosophy of memory.

Behind all of this stands an older voice. The eighteenth-century philosopher David Hume, searching his own experience for the self, reported that he could never catch himself, as he put it, without a perception, never find the I as a separate item, but only ever the passing thoughts, sensations, and feelings themselves. The self, Hume suggested, does not show up in experience as a thing over and above the experiences.

In the twentieth century, the philosopher Daniel Dennett offered a modern version of the same deflation, describing the self as a center of narrative gravity, not a physical object you could locate, but a useful abstraction around which a story organizes itself.

These are the strongest statements of the view that the self is a pattern rather than a substance. Whether a pattern is all you are is the question the picture cannot finally settle from inside itself.

The teletransporter

The same conclusion can be reached without any scalpel, through a thought experiment Parfit pressed in Reasons and Persons: teletransportation.

A machine on Earth scans your body to the last atom, records the exact arrangement, and in the scanning destroys it. The information races to Mars, where an assembler builds, from fresh matter, a perfect replica, molecule for molecule, carrying every memory, every intention, every half-finished thought you held as you stepped in. The replica wakes on Mars certain that it is you, remembering Earth, resuming your plans.

Is it you? Did you travel to Mars, or did you die on Earth while a stranger with your memories was born on another world?

The reductionist answer is calm and unsettling. The question of strict identity may have no determinate answer, and, more importantly, it may not be the question that matters. What matters, on this view, is whether Relation R holds, the right kind of psychological continuity and connectedness, carried forward by any reliable cause. If Relation R holds, then everything you ordinarily care about in survival is preserved, whether or not the metaphysical ledger says the person on Mars is numerically the same as the person who stepped in on Earth.

Notice how naturally that marries the four-dimensional picture. If you are a worm of temporal parts, or a succession of stages, then the teletransporter has performed neither a miracle nor a murder. It has merely relocated the pattern, the way a song is the same song whether it is played on this instrument or that.

The question the picture cannot answer

A and B theories of time, worm theories and stage theories, reductionism and animalism, Lewis and Sider and Parfit and Olson and Hume and Dennett, all of them argue from inside a shared assumption. The assumption is that the self is, at most, an arrangement of parts that physics or psychology can specify. The disputes are about which parts to count and how to count them.

There is one remainder the disputes do not address. You are reading these words as one person. Not as a committee of slices. Not as a sum of momentary parts. As one undivided awareness, gathering a sentence into a meaning, aware that it is you and no one else doing it. That unity is the most immediate fact you possess, more immediate than the chair beneath you or the light in the room.

What is doing the unifying?

Hume reports that he searched his experience and could never catch the self. But notice who is doing the searching. The looking, the failing-to-find, are acts, and acts have an agent. The introspection that fails to locate the self is performed by the self it cannot locate. Dennett’s center of narrative gravity is a story told and a narrative entertained, and a story requires someone to whom the telling appears. An illusion with no one deceived is not an illusion.

Whether the unified subject can be assembled from mindless temporal parts, or whether it must be the gift of something already conscious and already one, is not a question Sider’s Four-Dimensionalism sets out to answer. It is not a question physics is equipped to settle. The block universe charts the worldline. It does not say what gathers the worldline into one life.

This is the question every honest treatment of perdurantism eventually arrives at, and the question the companion documentary on the Sleepy Joe Space YouTube channel takes up at length.

What stays

What stays is the picture, laid out clearly. The block universe is the natural reading of Einstein’s relativity, and the temporal parts theory is the natural reading of persistence inside it. The puzzles of change, of coincidence, of fission, all dissolve more neatly on the four-dimensional view than on the everyday view of the wholly-present self. Sider’s defense remains the canonical modern statement, and most contemporary metaphysicians treat the picture as a serious live position even when they do not finally accept it.

What also stays is the remainder. The unified subject who reads these words, follows the arguments, and recognizes the puzzles as puzzles, has not yet been located inside any of the theories. Whatever the right account of persistence over time turns out to be, the I who persists is not just another item in the inventory of parts. It is the one to whom the inventory appears.

If the block universe is right and you are a worm of temporal parts, the worm has a witness. The picture does not explain the witness. It assumes it. And that assumption, followed honestly, points beyond the picture to something the picture itself cannot contain.