How do we know that time exists?

This is a complex question.

First, we cannot make sense of a question like this without first establishing what we mean by knowledge.

For convenience, let's pick the popular justified true belief (JTB) definition of knowledge. On this definition, the following conditions must be met in order for us to know that time exists:

  1. It has to be true that (some kind of) time in fact exists (for sake of argument, let's grant that this is the case)
  2. We must believe this is the case (pretty much everyone does, so let's grant this as well)
  3. We need to be aware of a justification for our belief in the proposition that time exists

And here lies the heart of my question: What justification do we have to believe that (some kind of) time exists?

One possible attempt at justification would be to appeal to our subjective personal experience of time (e.g. "I know that time exists because I have the subjective experience of perceiving things changing over time"), but then this kind of justification would be vulnerable to proving too much, since people could just as easily use the same justification to justify other controversial beliefs, such as:

  • Belief in ghosts, angels, demons, etc. (e.g. person X says "I know that ghosts exist because I have personally experienced/witnessed ghosts")
  • Belief in aliens (e.g. "I know that aliens exist because I have subjectively experienced aliens (I was abducted by aliens)")
  • Belief in past lives (e.g. "I have subjectively experienced visions of my past lives")
  • Belief in some specific religion (e.g. Mormons for example appeal to the witness of the Holy Ghost; William Lane Craig also makes a similar argument here)
  • Etc.

How can we justify our belief in the existence of (some kind of) time without "proving too much"?
Or should we just bite the bullet?

Regarding justification, this is probably related: Is there an objective standard of sufficient evidence?

  • 5
    Why time? The exact same argument applies to all knowledge we claim to have, not just the existence of time. Sep 8, 2022 at 17:10
  • 1
    "Some kind of" ghosts, angels, past lives, Holy Ghost, etc., all exist as experienced, be they imaginary/fictional or not. And so does experience of duration that we, in part, conceptualize as time, so acknowledging it does not prove too much. However, most people associate time not so much with that but with much firmer grasped and public physical events, like sunrises and clocks. When ghosts, angels and aliens become equally accessible they can compete with time in epistemic status, but not as it is.
    – Conifold
    Sep 8, 2022 at 20:15
  • 1
    As Conifold said, the subjective experience of time is both ubiquitous and consistent among people, but many people have never seen a ghost. Of course it presupposes we can trust our recollection of other's testimony, but if you go that way you simply can know nothing. Then why would you even be asking on Philosophy SE? What proves we other posters are real? Another thing is the lack of alternative hypothesis: when people see a ghost there is always the possibility that it was the wind, or a shadow, but what other explanation is there to the fact that we perceive events one after another?
    – armand
    Sep 9, 2022 at 0:10
  • 1
    Indeed contrary to the famous ingrained Kantian category of the most inner sense time intuition (even preceding the outer sense of space) to posit that time is the mere form of sensation because any sensation seems must be felt in succession, some speculative modern physics theory posits time may be only an emergent phenomena arised from the unfolding of some quantum wavefunction's frequency domain (space) evolved and felt only inside our universe... Sep 9, 2022 at 1:54
  • 1
    External intersubjective agreement is not the standard, it is just a side effect, the standard is public access (to sunrises and clocks, in this case). If subjective experience of duration was all there is there'd be much more controversy over its status, as we see with qualia generally. Introspection has no epistemic parity with perception exactly because of its tenuous and private character (even when there is some intersubjective agreement), which once even prompted Kant to opine that psychology can never be a science.
    – Conifold
    Sep 9, 2022 at 22:14

6 Answers 6


This is a basic question of epistemology. How do we know ANYTHING exists.

There are three basic methods:

  1. Rationalism -- establish what is the case by reasoning
  2. Direct knowledge -- we know what is, by immediate knowledge that is undeniable. There is a variant on this, in that we could have direct knowledge (intuitions) that are NOT certain
  3. Indirect inference -- this is the principle of indirect realism that science operates on. Popper's science methodology is the summary of how this is done.

Method 1, rationalism, has been narrowed to -- basically a null set of knowledge of reality. The fist major reduction of category 1 was in Kant's Critique of Pure Reason. Kant held that there were a few aspects of reality that could be established in category 1, but his go-to example was Euclidian Geometry, which we subsequently discovered first was not the only kind of geometry, then further that our world is not Euclidean after all. The pluralism of logic https://www.cambridge.org/core/journals/think/article/guide-to-logical-pluralism-for-nonlogicians/EDFDFA1C9EB65DB71848DABD6B12D877 basically makes any "proof" intrinsically not provable.

Method 2, direct and certain knowledge, has been limited post Descartes to some limited aspect of perception, experience, or agency. Weaker possible knowledge could have a larger suite of features than this, but does not offer certainty.

Method 3, inferential knowledge, appears to be almost the entirety of what we "know".

Note, method 3 does not satisfy your presumed criterion of Justified True Belief. First, for indirect realism, one has no access to "truth" (and logic pluralism makes "truth" relative to a postulated logic anyway), so nothing can satisfy the T of JTB. Also, per the Munchausen Trilemma, no justification can itself be justified without eventually reverting to a fallacy. See this answer on the Trilemma: Is the Münchhausen trilemma really a trilemma? So no beliefs can be true, or justified.

What empiricists use instead as a criteria is usefulness. If a model is part of a vigorous and useful research programmed, which has been successfully answering long standing problems, then one can reasonable infer the reality of what is being modeled. This is Imre Lakatos's Research Programme methodology: http://people.loyno.edu/~folse/Lakatos.html

We know time is real because we experience it (this is one of the undeniable issues for direct realism), AND it is highly useful in logic based understanding of our universe (time, in the form of logic sequence of state changes, is needed for causation, which we basically need to even conceive of our universe), AND it is highly useful in the presentism form to understand motivations and events, AND in its block time version it is needed to understand physics, AND in its growing time mode, it is needed in all applications of history.

"Prove too Much", which your link claims is a demonstrated fallacy, is not a fallacy for any of your listed cases.

  • If I had experienced being captured, transported, and probed by aliens, I hold it would only be fallacious reasoning if I then did NOT believe aliens were real! Note indirect realism operates off of subjective observations, and most of what we consider "real" is only verified by first person empiricism. This is also an area where intersubjectivity can be applied.
  • I HAVE experience one past life, and reincarnation studies are a field where intersubjectivity HAS been applied. I have also communicated with ghosts, which is also a field where intersubjectivity can be applied. It is hardly "proving too much" for me to believe in both realities. Claims that one cannot ever infer that ghosts or past lives are real, are irrational dogmatism, and would themselves be a fallacious claim.
  • A religion is a complex worldview, which cannot be supported by a single observation. It CAN be evaluated by the same methodology that a scientific Research Programme is evaluated. If the Religion is shown to be highly useful in answering questions that other frameworks cannot answer, and its adherents admit to the problems that it cannot answer, but are making progress incrementally in answering them, then a Religion be reasonably assumed to be real. I don not believe any religion satisfies this criteria today, and instead they would all be considered regressive Research Programmes, per Lakatos' criteria. Which makes this only a theoretical concern, not an actual one.

So -- time exists, per the pragmatic standard of utility. So might all of the "prove too much" examples you cited.

  • Wait, so you're saying anyone who's has experiences with aliens, past lives, religious deities, etc. are all rational in believing those things to exist (even if they contradict one another)? If so: Do you believe objective reality exists? Would you agree that rationality roughly has the goal of getting our beliefs as close as possible to objective reality? If yes, how do you consolidate the fact that your epistemology explicitly allows for different people (or the same person) to hold contradictory beliefs, yet only a single objective reality exists?
    – NotThatGuy
    Sep 9, 2022 at 9:45
  • @NotThatGuy -- Humans are hard-wired to want to believe in simple ideologies. Empiricism holds that evidence trumps psychologically comforting theory. Yes, it is literally insane to deny overwhelming evidence in order to preserve an ideology. It was insane for the Catholic Church to deny the evidence of Galileo's telescope. It is literally insane for delusionists to deny the existence of consciousness. See this review of Blackmore: amazon.com/gp/customer-reviews/R1C1TJFIWBZ8ZQ/…
    – Dcleve
    Sep 9, 2022 at 10:39
  • Empiricism, and its children science and engineering, accept that our models and theories of the universe may not mesh. Developing a "theory of everything" is a project, not a currently successful accomplishment. And evidence trumps any effort to over-universalize any theory.
    – Dcleve
    Sep 9, 2022 at 10:42
  • "Developing a 'theory of everything' is a project, not a currently successful accomplishment" - but you're explicitly choosing to avoid consolidating different experiences people have (and epistemologies that would consolidate them DO exist), and your position seems to roughly be "everyone should believe whatever they see", which clearly rejects any attempt to develop a "theory of everything", and also seems to deny epistemological discussion as a whole. Or am I missing something?
    – NotThatGuy
    Sep 9, 2022 at 11:02
  • @NotThatGuy -- no, there are no ontologies that successfully consolidate all our knowledge. Its not even true in physics, where physical reductionism cannot currently consolidate QM, relativity, and solid state physics. This failure of the "universalizing " project in science, is why science has now abandoned it, and embraced pluralism. See section 5 of SEP: plato.stanford.edu/entries/scientific-reduction Pluralism, by definition, means that we cannot have a single overarching model.
    – Dcleve
    Sep 9, 2022 at 11:19

Not sure if I'm correctly understanding the crux of the question but the simple answer seems to be that time is a necessary coordinate in a coordinate system. You can't have location without time. (Usually "now" is implied but isn't necessarily the case.)

We tend to think of things having a location of, say, latitude, longitude and altitude. But if you really want accuracy, you must include time as a coordinate. My eyeglasses were on my nightstand at coordinates of X,Y,Z,t. The "t" necessarily defines what time my eyeglasses were at that X,Y,Z location. Missing the "t" is just like missing any other coordinate. If my eyeglasses are at X,Y,t with unknown Z (where Z is altitude) then maybe my eyeglasses were in the basement. Or in the living room above the basement. Or my upstairs bedroom. Or on the roof. Or on an airplane overhead. Missing the Z means we don't know the altitude of my eyeglasses. Missing the t means we don't know what time they were at the X,Y,Z and is therefore also not very useful.

So time must exist because without it we lack a critical coordinate.

(If you also think of space and a coordinate system relative to the galactic center then earth's location is changing in X,Y,Z,t. If you change the t, then the X,Y,Z would be in a different place. If time travel was actually a real thing, the first time travelers would have a terrible problem as they go back in time at the same X,Y,Z, to the year 1860, and find themselves stranded in space because the earth was literally trillions of kilometers away from that X,Y,Z at that t. Hope your time travel machine is also a space ship. Dr. Who would presumably be okay.)

  • I fail to see the logic in this answer. Your argument can be summarized as "if a belief is useful (according to an unspecified criterion of 'usefulness') then it must be true". Well, for many people the law of attraction is useful, therefore the law of attraction exists?
    – user48437
    Sep 8, 2022 at 18:29
  • Rephrase your question to ask "How do we justify our belief that altitude exists" and you have the same question, and the same answer. You think time is a wishy washy concept. It's just a coordinate in a 4 dimensional space. The logic is that some things are objective truths, or at any rate, I would like to see your logical proof of altitude, because I am one word swap away from using that to prove time.
    – JamieB
    Sep 8, 2022 at 18:38
  • The logic is that some things are objective truths - This is not a logical argument, this is just a claim. Claims are not arguments. I would like to see your logical proof of attitude - I never claimed that altitude exists, so I have no burden of proof in that regard.
    – user48437
    Sep 8, 2022 at 18:40
  • The problem you are missing, as we see in all of your examples, is that time is not a "personal experience" in the way you are spelling out. It may be relative, but it is still just a coordinate. It's not a ghost or an alien or a religion or a past life. Your experience of time is irrelevant to the fact that time objectively exists as a coordinate. You are ultimately asking us to justify our belief that things have coordinates.
    – JamieB
    Sep 8, 2022 at 18:50
  • Your experience of time is irrelevant to the fact that time objectively exists as a coordinate - How do you know that time objectively exists as a coordinate? You are ultimately asking us to justify our belief that things have coordinates. - If time is defined as a coordinate, then sure, I'm asking that.
    – user48437
    Sep 8, 2022 at 18:53

How do we know that time exists?

This requires establishing what we mean by exists.

Belief requires thoughts differentiated by time, the motion of thought. Thought reflecting back on itself.

Quoting Derrida from Heidegger: The Question of Being and History, page 184:

describing time as pure auto-affection, Heidegger writes,

According to its essence, time is pure affection of itself. Furthermore it is precisely what in general forms [aiming: intuition, the way] seeing which, setting off from itself, heads for . . . [which translates so etwas wie das, “Von-sich-aus-hin-auf-zu”] something like the “from-out-itself-toward-there . . . ,” so that the upon-which looks back and into the previously named toward-there. (Kant and the Problem of Metaphysics, 132)

This form of temporality is intrinsic to the being of a human, and the human in conjunction with phenomena constitute the existence of phenomena, such as the physical measurement of time.

  • Can't you similarly say {insert experiences of a god-like entity here} is "intrinsic to the being of a human"?
    – NotThatGuy
    Sep 8, 2022 at 20:30
  • Experience of reality could be said to be miraculous. Sep 8, 2022 at 20:31

Here is my take on this.

The existence of history is a strong indicator that time exists. For example, we can study tree rings, ice cores, alluvial deposits, stalagtite growth marks and the like to demonstrate that plant growth, seasonality and climate all existed long before humans learned how to record their own histories in words.

Furthermore, excellent evidence of the passage of time is obtained through the use of space-time diagrams which furnish accurate predictions of how different observers will experience reality in different coordinate frames.

  • For example, we can study tree rings, ice cores, alluvial deposits, [...] - How do we know that tree rings, ice cores, alluvial deposits [...] exist? excellent evidence of the passage of time is obtained through the use of space-time diagrams - how do we know that space-time diagrams exist?
    – user48437
    Sep 8, 2022 at 21:10

Appealing to our subjective personal experience of time is perfectly fine, and doesn't prove too much (but you're only justified in believing the simplest explanation for the evidence).

Given our subjective experience, the existence of time is a simpler explanation than the non-existence of time.

Regarding experiences of ghosts, angels, demons, aliens, past lives and religious beings, those can all have a simpler explanation of hallucinations, dreams, misinterpretations of one's experiences, etc. As we gain a better understanding of reality, life, death, our bodies, our brains, the universe, our origins, beliefs of different cultures, etc., fabrications of our minds becomes a more simple explanation, while actual supernatural beings becomes less simple due to a lack of strong supporting evidence and how their existence might conflict or be difficult to explain with our current understanding of reality based on the available evidence.

We already know people can interpret their experiences in a way that conflicts with reality. Yes, this includes hallucinations and dreams, but also optical illusions, seeing a monster that turns out to just be a coat rack, etc.

Regarding time, there is lots of supporting evidence and little to no contrary evidence: you can see evidence of time's existence in reality, others can recount their experience of time for you, our own mental processes indicates the passage of time, etc. These are all subjective experiences, yes, but they all still contribute to making the existence of time the simplest explanation.

If you see a monster, you turn on the light, and then you see a coat rack, this would be strong contrary evidence for the existence of the monster you saw.

To demonstrate why believing only the simplest explanation is justified, consider this simple example:

You see an empty can on a table. In the absence of any other evidence, do you conclude that:

  1. A person put it there.
  2. A person put it somewhere else on the table, and the wind blew it to where it is now.
  3. The can materialised from thin air.

I would hope you went with #1. The problem with #2 is that it adds an additional unnecessary assumption (person + wind, instead of just person). The problem with #3 is that it relies on something that we have no evidence has happened before, whereas #1 happens all the time. #1 is much simpler than both #2 and #3.

  • but you're only justified in believing the simplest explanation for the evidence - 1) Why? Where did this rule come from? 2) How do you measure simplicity? Regarding experiences of ghosts, angels, demons, aliens, past lives and religious beings, those can all have a simpler explanation of hallucinations, dreams, misinterpretations of one's experiences, etc. - We can say the same thing about everything pretty much: every experience has the simpler explanation of just being a hallucination, misinterpretation, etc.
    – user48437
    Sep 8, 2022 at 21:27
  • To demonstrate why believing only the simplest explanation is justified, consider this simple example - Is what follows after this sentence an inductive, deductive, abductive argument, or something else? The problem with #2 is that it adds an additional unnecessary assumption (person + wind, instead of just person) - but option #1 also adds the assumption of no wind, right?
    – user48437
    Sep 8, 2022 at 21:29
  • The problem with #3 is that it relies on something that we have no evidence has happened before, whereas #1 happens all the time. - Intuitively it seems to make sense, but in this case this is not part of the experience itself. In fact, your whole example is about hypothesizing how the can could have appeared on the table, not about the nature of your perception of the can and the table themselves (the subjective experience itself). If you are justified to believe that there is a can on top of a table, aren't you also justified to believe that aliens abducted you if you experience it?
    – user48437
    Sep 8, 2022 at 21:37
  • @SpiritRealmInvestigator "in this case this is not part of the experience itself" - yes, you should consider all knowledge/evidence you have about reality when evaluating any given claim/experience. "your whole example is ... not about the nature of your perception of the can and the table themselves" - yes, but the same principle applies. You would justify you belief that there's a can on the table in a similar way (you seeing what's there is simpler than e.g. you being a brain in a vat), and after you've done that, you can conclude an explanation for how it got there.
    – NotThatGuy
    Sep 9, 2022 at 8:15
  • 3
    It is always "simpler" to deny any evidence that contradicts one's preferred theory. Of course, the "simplest" theory of all is that everything one experiences is an invention of one'sw own mind. This ideological pursuit of "simplicity" is a dead end.
    – Dcleve
    Sep 9, 2022 at 10:55

We can observe that things change. Change requires that a thing/system O be in one state A and then another state B such that A != B. Since O takes on states A and B, but A != B, then O cannot simultaneously be in both states A and B. The only resolution is that O take on distinct states non-simultaneously, aka at different times. We know time exists because things change.

  • 1
    You could also (/ more accurately) apply this to our observation of O (rather than O itself), which would allow us to conclude that time exists regardless even without having to know whether O really exists (time would, at the very least, exist within your mind ... or within whichever entity generates your thoughts). Although you may technically not know whether the past actually exists, so this may not entirely solve the problem.
    – NotThatGuy
    Sep 9, 2022 at 10:44
  • We can observe that things change - Sure, but this is exactly the reason I mentioned in the OP that ends up "proving too much".
    – user48437
    Sep 9, 2022 at 11:48
  • @SpiritRealmInvestigator Not clear on what you mean by "proving too much" - it only proves time exists Sep 9, 2022 at 17:55
  • en.wikipedia.org/wiki/Proving_too_much
    – user48437
    Sep 10, 2022 at 0:49
  • @SpiritRealmInvestigator Thanks for the reference. How does it apply here? Sep 10, 2022 at 21:13

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