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There is a peculiar kind of vertigo that comes from asking what time actually is. We use it constantly — we schedule it, waste it, save it, run out of it — and yet the moment we try to define it, it slips away from us. Augustine confessed this problem sixteen centuries ago: he knew what time was, he said, until someone asked him to explain it, and then he found he knew nothing at all.

Philosophy has spent the centuries since trying to answer Augustine’s question properly. Not “how do we measure time,” which is a problem for physicists and clockmakers, but something stranger and more basic: does time… actually pass? Is the present moment special, or is it just one location among many? Is space a real container that things sit inside, or merely a word we use for the relationships between things?

These questions sit at the foundation of metaphysics, and they turn out to be far less settled than most people assume.

This piece is an attempt to lay that foundation clearly — what the major positions are, where the weight of argument currently sits, and where the real disagreements remain. Once that ground is established, I want to use it to introduce a further idea I’ve been developing: a view I call the Spilled milk theory, which doesn’t just take a side in these debates but tries to dissolve the question that produces them in the first place.

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꩜ The Oldest Disagreement: Is Space a Thing, or a Relationship?

The first fault line in this territory opened up between two of history’s sharpest minds: Isaac Newton and Gottfried Leibniz.

Newton needed space and time to be real in their own right — actual entities, independent of anything happening inside them. His physics depended on absolute space (a fixed, unmoving backdrop against which true motion could be measured) and absolute time (a uniform flow, ticking forward identically everywhere, whether or not anything existed to do the ticking). On this view, called substantivalism, if you removed every object in the universe — every particle, every star, every observer — space and time would remain. They would simply be empty.

Leibniz thought this was nonsense. For him, space was nothing more than the order of coexisting things — the set of relations between objects — and time was nothing more than the order of successive events. This view is called relationalism. On this account, “empty space” isn’t a thing waiting to be filled; it’s an incoherent phrase, like “marriage” without any spouses. If you removed every object in the universe, there would be no space left over, because space was never an entity — it was always just a pattern of relationships, and you can’t have a pattern with nothing left to relate.

This wasn’t an abstract dispute. Newton had an empirical argument on his side: the famous bucket experiment. Spin a bucket of water, and the water’s surface curves upward at the edges — even once the bucket and water are rotating together, with no relative motion between them. Something is happening, Newton argued, and that something can’t be explained by relations between the water and the bucket, since they’re not moving relative to each other. The only thing left to be rotating relative to is space itself. Absolute space, it seemed, had to be real to explain the curve of the water.

Relationalists have spent three centuries trying to answer this challenge, and the debate hasn’t fully closed. Einstein’s general relativity complicated the picture further still: spacetime in general relativity is dynamical — it bends, stretches, and ripples in response to mass and energy, which makes it look less like Newton’s passive container and more like a genuine participant in physical events. Some philosophers take this as a vindication of substantivalism (spacetime is now clearly a “thing,” since it can be acted upon and can act). Others read general relativity as relationalism in disguise, since what’s truly physical, on this reading, are only the relations encoded in the metric field, not some extra entity sitting beneath them.

The honest summary: this debate is alive, technical, and unresolved. It has not been settled by physics in either direction, despite occasional confident claims to the contrary.

It’s worth pausing on why this particular disagreement has so much staying power, because it isn’t merely academic hair-splitting. What’s actually at stake is the relationship between structure and substance. If space is a substance, it has its own properties, its own causal powers, its own facts about it that hold independently of what’s inside it — it becomes, in effect, one more entity added to your inventory of what exists. If space is purely relational, then talking about “space” at all is really just a convenient shorthand for talking about how things stand with respect to one another, and there is, strictly, one fewer fundamental kind of thing in the universe than the substantivalist believes. This is not a trivial difference. It changes what you think the most basic furniture of reality actually consists of, and by extension, it changes what a complete inventory of “everything that exists” would need to include.

Leibniz had a second argument worth mentioning, distinct from the relational analysis of space itself: the principle of sufficient reason, the idea that nothing happens, and nothing is the way it is, without some reason that it is that way rather than some other way. Leibniz pointed out that if space were an absolute, independently existing container, God (or nature) would have had to make an arbitrary choice about exactly where to place the entire material universe within that infinite, featureless container — three feet to the left, or three feet to the right, makes no detectable difference to anything, since there would be nothing else to measure the shift against. But an arbitrary choice with no possible reason behind it struck Leibniz as philosophically intolerable. If, instead, space is merely the relations between things, the problem evaporates: there is no question of “where” the universe as a whole is located, because location was never anything more than relative position to begin with, and the universe can’t be relocated relative to itself.

Newton’s defenders, through his associate Samuel Clarke, pushed back hard in a famous correspondence with Leibniz, and the bucket argument was their strongest card. It is genuinely difficult to explain the curved water surface using only the relations between the bucket and the water, since those relations don’t change throughout the experiment, yet the water’s behavior does. Relationalists have proposed various solutions over the centuries — some appealing to the relation between the water and the rest of the matter in the universe (the “fixed stars”), a move associated with the physicist Ernst Mach and known as Mach’s principle — but none has commanded the kind of universal acceptance that would simply end the debate.

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꩜ Does Time Pass? Three Competing Pictures

If the substantivalism debate is about space, the parallel debate about time is, if anything, even more central to how people live. It concerns whether time has a privileged “now,” and whether the past and future are real in the same sense the present is.

There are three major positions.

Presentism holds that only the present moment exists. The past has slipped out of being; the future hasn’t yet come into being. This is close to common intuition — when we say “the dinosaurs no longer exist,” we usually mean something stronger than “the dinosaurs are not nearby.” Presentism is also the most ontologically economical view: it commits you to the existence of exactly one moment, not an infinite stack of them.

Eternalism, sometimes called the block universe view, holds the opposite: every moment in time is equally real. Past, present, and future are not different in kind — they differ only in their position, the way “here” and “there” differ in space. On this view, your birth and your death are both real events sitting at fixed coordinates in a four-dimensional structure; “now” is not special in the structure of reality, it’s just the particular coordinate you, the observer, happen to be located at. There is no objective flow; there’s just the whole block, all at once, and consciousness moving through it the way a flashlight beam moves across a wall that was already fully painted.

The growing block theory tries to split the difference. It agrees with eternalism that the past is just as real as the present — once something happens, it’s permanently part of reality’s structure. But it agrees with presentism that the future is genuinely open and does not yet exist. Reality, on this view, is a block that is constantly growing at one edge: the present is the leading edge of being, where new reality is continuously laid down.

What makes this more than an academic parlor game is that physics has weighed in, and not gently. Special relativity shows that simultaneity is relative to an observer’s reference frame: two observers moving at different velocities can disagree about which of two distant events happened “at the same time,” and neither is mistaken. There is no privileged, universal “now” built into the structure of relativistic spacetime. As discussed above, many philosophers — though not all — take this to be a serious problem for presentism, since presentism needs there to be one objectively correct present moment, and relativity seems to deny that any such moment exists. This is part of why eternalism has become the dominant position among philosophers of physics, even though it remains contested by philosophers who think the block universe can’t account for the felt reality of time’s passage, the special status of “now,” or the asymmetry between a fixed past and an open future.

It is worth being precise about what “consensus” means here, because the word gets used loosely. There is a real and well-documented lean toward eternalism among professional philosophers of time, largely driven by relativity. But “lean” is not “unanimity,” and serious philosophers — not cranks, not laypeople — continue to defend presentism and the growing block view, arguing that relativity describes the geometry of measurement, not necessarily the deep metaphysics of being. Anyone telling you this question is closed in either direction is overstating their case.

There’s a further wrinkle worth knowing about, because it shows how technical this debate can get once you look past the headline positions. Presentism faces what’s sometimes called the truthmaker problem. If only the present exists, what makes a sentence like “Napoleon lost at Waterloo” true today? The event itself, on a strict presentist reading, no longer exists in any sense — so there is nothing left in reality to make that past-tense sentence true. Eternalists have an easy answer here: Napoleon’s defeat is just as real as anything happening right now, sitting at its own fixed coordinate in the block, available to serve as a truthmaker for the sentence. Presentists have had to do real philosophical work to answer this challenge, generally by proposing that the present moment carries traces, properties, or “tensed facts” about the past within it, even though the past events themselves are gone. Whether this maneuver fully succeeds, or merely relocates the problem, remains an active and genuinely difficult question — not a settled embarrassment for presentism, but not a fully resolved difficulty either.

There’s also a deeper motivation for caring about this debate beyond physics and logic puzzles, one that’s easy to miss if you only look at the formal arguments: the positions carry real implications for how we think about morality, regret, and the openness of the future. If eternalism is true and the future already exists in the same sense the present does, it becomes harder to make sense of the intuition that the future is genuinely undetermined — that things could still go more than one way. This connects the metaphysics of time to old and unresolved debates about free will and determinism, which is part of why the dispute has never simply faded into a settled, technical footnote. People have a stake in the answer that goes beyond curiosity about the furniture of the universe.

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꩜ McTaggart’s Strange Argument That Time Might Be Unreal

No introduction to space & time is complete without J. M. E. McTaggart, a British idealist philosopher writing in the early twentieth century, who made one of the strangest and most influential arguments in the entire field: that time, as ordinarily conceived, does not exist at all.

McTaggart’s move was to distinguish two ways of describing temporal position.

The A-series describes events using tensed, perspective-dependent language: an event is past, present, or future. These properties are not fixed — they change. Your reading of this sentence is, right now, present; a moment from now, it will have slipped into the past. The A-series is how we naturally talk and think about time, and it’s where all the “flow” and “passage” intuitions live.

The B-series describes temporal position using a fixed, tenseless ordering: event X is earlier than, simultaneous with, or later than event Y. These relationships never change. The signing of a particular treaty will always be earlier than a particular war that followed it, regardless of which moment you’re standing in when you say so. The B-series is essentially what a block-universe eternalist thinks is the whole truth about time.

McTaggart’s argument, compressed considerably, was that the A-series is incoherent, because it requires every event to possess contradictory properties: each event must be future, then present, then past, and these are supposed to be mutually exclusive properties that nonetheless all apply to the same event over its “history.” Trying to resolve this by saying an event is future at one time and past at another just reintroduces the same problem one level up, because now you need to ask whether that further time is itself future, present, or past — leading to an infinite regress that never resolves the contradiction. Since the A-series is incoherent, and McTaggart thought the A-series was essential to time as ordinarily conceived (genuine change requires real passage, not just a static ordering), he concluded that time itself, in the sense most people mean by the word, does not exist.

Hardly anyone fully accepts McTaggart’s conclusion that time is wholly unreal. But the argument forced an entire field to get precise about something it had previously taken for granted, and the A-series/B-series distinction remains one of the standard tools for analyzing every other position discussed above. Presentists are, in effect, defending the reality of the A-series. Eternalists generally hold that the B-series is the whole truth, and that the A-series — the felt difference between past, present, and future — is a feature of consciousness, not of the world itself.

McTaggart also described a third, thinner notion he called the C-series: simply the ordering of events, without even the tenseless “earlier/later” relations of the B-series attached, closer to a sequence of items on a list whose order matters but whose direction does not. The point of introducing it was to isolate exactly how much temporal structure survives if you strip away first the A-series, then the directional asymmetry of the B-series. What’s left, on McTaggart’s own reckoning, is something more like a static ordering than anything we’d ordinarily recognize as time at all — which was precisely his point. If genuine time requires the A-series, and the A-series is incoherent, then what’s left over, however you label it, doesn’t actually deserve the name “time.”

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꩜ What Relativity Actually Changed

It’s worth being concrete about what special relativity showed, since it gets invoked constantly in these debates and is rarely explained.

Before Einstein, the assumption — formalized by Newton but shared by nearly everyone’s intuitions — was that there is a single, universal “now” shared by the entire universe. Two events either happen at the same time or they don’t, full stop, and everyone everywhere would agree about which. Special relativity broke this. It showed that whether two distant events are simultaneous depends on the observer’s state of motion. An observer on a fast-moving train and an observer standing still on the platform can each correctly determine that two distant flashes of light occurred simultaneously, and disagree with each other — not because one of them made an error, but because simultaneity itself is not an absolute, frame-independent fact. It is relative to a reference frame, in exactly the way that “to the left” is relative to which way you happen to be facing.

This is the result philosophers lean on when they argue against presentism. Presentism needs a single, objectively correct present moment — a genuine fact about which events are happening “now,” independent of any observer. Special relativity seems to deny that any such fact exists. There is no view from nowhere that picks out one universal “now”; there are only different observers’ equally valid present moments, disagreeing with each other about which distant events belong to it.

Some presentists respond that this is a problem about what we can know or measure, not about what’s metaphysically true — perhaps there really is one objective present, even if relativity makes it permanently undetectable to any observer limited by the finite speed of light. This is a coherent position, but it comes at a cost: it requires positing a fact about the universe that is, in principle, forever hidden from any possible observation, which sits uneasily with how science usually prefers to operate. Whether that cost is worth paying is, again, a live and unresolved dispute rather than a closed case.

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꩜ Why Entropy, Not Metaphysics, Usually Explains Time’s Arrow

There’s one more standard piece of the puzzle worth including in any intro: why does time seem to move in one direction?

The fundamental equations of physics are, for the most part, time-symmetric — they work equally well running forward or backward. A film of two billiard balls colliding looks physically plausible whether you play it forward or in reverse. But a film of an egg breaking against a floor looks obviously wrong played backward. Why does the universe seem to have a direction, an arrow, if the underlying laws don’t prefer one?

The standard answer comes from thermodynamics. The second law of thermodynamics states that entropy — roughly, the degree of disorder or the number of ways a system’s microscopic details could be arranged while still looking the same at the macroscopic level — tends to increase in closed systems over time. An intact egg is a low-entropy, highly ordered state with relatively few ways to arrange its components and still have it look like “an intact egg.” A broken egg is high-entropy: there are vastly more ways to scatter shell fragments and yolk that all look like “a broken egg.” Disorder is statistically far more probable than order, so systems evolve from the rare ordered configurations toward the overwhelmingly more common disordered ones. This statistical asymmetry is generally taken to explain the arrow of time we experience.

This leaves a residual puzzle that philosophers continue to debate: does entropy explain why time feels like it’s flowing, with a lived, present-tense quality to each passing moment — or does it only explain why physical processes are asymmetric, leaving the subjective experience of flow as a separate question entirely, one that thermodynamics was never equipped to answer in the first place?

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꩜ Where This Leaves Us

Step back and look at the shape of all four debates together, and a pattern emerges. Substantivalism versus relationalism asks whether space is a container or a relationship. Presentism versus eternalism asks whether the present is metaphysically special or just one position among equally real others. McTaggart’s argument asks whether the felt passage of time can be made logically coherent at all, or whether it’s a structural illusion riding on top of a static, tenseless ordering of events. The entropy account of time’s arrow explains why physical processes look asymmetric, but stops short of explaining why there is something it is like to experience that asymmetry as a flow.

Notice what’s common across every one of these unresolved residues. In each case, the open question is not really about the external structure of space and time — physics has quite a lot to say about that structure, and says it with precision. The open question is about the relationship between that structure and an observer who experiences it. Eternalists need to explain why a tenseless block produces the vivid impression of a privileged “now.” Entropy theorists need to explain why increasing disorder produces the felt quality of flow, rather than just an objective statistical trend. McTaggart’s regress only bites because each “time” in the regress is implicitly a time for someone — a perspective from which an event is observed as future, then present, then past.

Consciousness keeps showing up as the uninvited variable in a debate that was supposed to be purely about the external world.

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꩜ The Spilled Milk Theory

Every position surveyed above shares a hidden assumption: that space and time are the stage, and consciousness is, at best, an actor that happens to walk across it — or, at worst, an afterthought, a biological side-effect that emerges somewhere inside the stage once enough matter gets arranged in the right way. Substantivalists think the stage is a real object.

Rather than treating consciousness as something that eventually appears inside a pre-existing space and time, the Spilled milk theory treats space and time themselves as a kind of unfolding of consciousness — not a container that experience happens to occupy, but something closer to a spill, an outpouring, with space and time as the very shape that the spilling takes. On this view, asking “how does matter produce experience” — the so-called hard problem of consciousness — is a bit like asking how a river produces wetness. The question assumes a separation between two things that were never actually separate to begin with. If experience is the substrate, and space and time are how that substrate unfolds and differentiates itself, then the hard problem isn’t solved so much as it dissolves: there is no gap left to cross, because matter was never the more fundamental term in the first place.

Once you’ve sat with eternalism’s puzzle of the felt “now,” and entropy’s puzzle of felt flow, and McTaggart’s regress of perspectives nested inside perspectives, the recurring shape of the residue starts to look less like noise and more like a signal. Each tradition, working entirely on its own terms, keeps running into the same wall: the structure of spacetime can be described with total precision, and still something is left over, and that something is always, suspiciously, experience itself.

The Spilled milk theory takes that residue seriously rather than treating it as a leftover to be explained away later. It asks what happens if we stop assuming experience is the late arrival and consider, instead, that it might be the earliest thing there is — the very substance out of which “early” and “late,” “here” and “there,” “space” and “time,” first spill into being.

I’ll continue that thread, here.