I am trying to wrap my head around different philosophical concepts of 'forever'. Correct me if I'm wrong, but I believe there to be three versions of 'forever' in terms of temporal duration:

  1. Strictly eternal, timeless and atemporal: there is no time.
  2. Actually sempiternal, or actual infinite temporal duration: there is time but it has neither beginning or end (as per Aristotle).
  3. Potentially sempiternal, or potential infinite temporal duration: there is time and it has a beginning but has no end; the temporal duration from the beginning to now is always finite, but the infinite addition of time makes it potentially infinite.

There is also the 'non-forever' case of truly finite temporal duration in which time has both a beginning and an end.

What is puzzling me is what is time that has an infinite past but will come to an end. The temporal duration of the past is actually infinite, but the fact that will come to an end makes it potentially finite. If we cannot know the future and therefore cannot measure future duration, is it infinite until it's not in the same way that case 3 above is finite until it's not? The main difference is that one never gets to crossover to the infinite in case 3, but one does cross to finite time in this last case.

  • If we are enumerating all possibilities, maybe there would be also no beginning, but an end - which is a really weird one. Don't know if it makes sense. At the same time, enumerating possibilities like this feels like we are ignoring or altering the actual semantics of "time" - it just becomes an extended quantity which can be bounded on at most 2 sides. Does that capture enough of the essence of "time" as we know it?
    – Frank
    Jan 10 at 4:21
  • "If we cannot know the future and therefore cannot measure future duration, is it infinite until it's not"??? If we cannot know X it does not mean that it is Y (or not Y) for any Y. And what is the difference between "potential" and "actual" infinite in this context? Is the idea that the past is set in stone so if it is infinite then actually infinite, while the future is open, so can be anything only potentially (this would not be a standard use of "actual/potential infinity")? Also, how is time having an end any more puzzling when it does not have a beginning than when it does?
    – Conifold
    Jan 10 at 5:29
  • Thanks, @Frank. I understand that the enumerated possibilities have all been subject to philosophical questioning and a finite or infinite temporal duration are considered to be possibilities. So I don't think it is easy to say that it is 'ignoring or altering the actual semantics of "time"' because a sense of both the finity and the infinity of time have long been part of the discourse. Jan 10 at 8:10
  • Thanks, @Conifold. Of course, It can be the case that if we cannot know X that is Y for any Y, and this is the case here: unknowns cannot be measured. Jan 10 at 8:13
  • An actual infinite is, I believe, uncountable, and cannot be achieved by addition. Whereas a potential infinite is finite with an infinite number of additions. Jan 10 at 8:14

5 Answers 5


Infinity is a tricky concept, and a lot of work has been done by mathematicians to make it logically consistent. I don't claim to understand it myself, but I can spot a potential non-sequitur in your final paragraph, where you mention time with an infinite past and a finite future being finite overall. On what ground do you suggest that? Suppose you take the set of positive numbers, which is infinite. If you conceptually reverse the set, counting back from infinity you might consider that you will eventually reach some number, say 2017, from which there is only a finite number of integers left before you reach the end of the set at 1. Does that make the set finite?

  • Thanks, @Marco Ocram. I am much happier thinking of a finite past with an infinite future, which is often done. The duration between the beginning of time to now is always finite. It is like counting the set of positive numbers from 1 upwards without stopping: you are always at some finite number, but the process is infinite. I'd expect the reversed case to have some symmetry with that one but it seems absurd while not inconceivable. Can you count down from infinity? Yes, if it's countable. Jan 10 at 8:21
  • I think the case where time has an infinite past but a finite end is a real puzzler.
    – Frank
    Jan 10 at 15:10
  • @frank I'm not sure what your objection is. An infinite set can have an end-point. That said, I think the question about whether time is infinite is secondary to the question about what could determine whether time is infinite. Suppose, hypothetically, that the Universe settled into some state of equilibrium- what would prevent it from enduring forever? Jan 10 at 16:35
  • I am not objecting, just commenting that from the 4 possibilities, the infinite-past-finite-future possibility seems to harder to imagine. Not saying it's not possible, just saying it doesn't seem so intuitive. For the question of "what could determine", I think we should send that to physics rather than philosophy for a definitive answer that will (eventually) count as more than speculation? We can generate hypotheses to test here of course, but the ultimate confirmation would have to be experimental.
    – Frank
    Jan 10 at 17:55

It's been helpful for me to think about it this way: that "time isn't real". The concept of "time" only emerges when we measure changes. Time is not a thing in itself. Without changes, there is no time, and with every change there is time by definition.

Our concepts of "past" and "future" are even less real. The past is a former state of things, and the future is a future state of things. It is possible to go "back" to the past by rearranging every piece of matter and energy in the universe to recreate it. However, this is not feasible due to the law of entropy.

So to the question — if the universe ceases to change, then time stops. If it starts changing again after this, then time begins again. In this model it's possible for time to have had a beginning, if the universe had never changed and then suddenly began to change.

We can even extend this principle of time to the supernatural, which may interest people who are interested in the topics of heaven and God and eternity in that sense. We can hypothesize about things outside of our universe that have other things besides matter and energy. This may alter our definition of time (for example in the Theory of Relativity, time is linked to space and matter) but the broadened meaning of "time" will still be a type of change, just not a change of matter and energy.

In the supernatural realm there may also be beings or "persons" ("person" means having a personality and will and thoughts, etc). (Side note: this is the sense in which people refer to a "personal" God). If these beings think or speak, then it is change because there is a before and after.

So even in the supernatural realm there could be something like time, though it would not necessarily be related to space, matter, and energy.


Consider this sentence from an (imagined) book in a universe that, as a whole, is in your second category (infinite past, infinite future):

"The stone had been there forever, all of eternity standing in the same spot, until Gareth had come and knocked it out of its place".

If we consider this, we see that we have an example of a time period that has an infinite past but an end. We also see that, no matter when in that past we are, there is only finite time until this "end", just like there is always a finite time since the beginning in a universe with a beginning but no end.

You never "cross over to finite time", no matter the point in time, the past is eternal, and the future finite. You also never "cross over into infinite time" in case 3, the past is always finite and the future infinite, regardless of what point of time we are talking about.


I believe you're looking for this: Ananta

  1. Anadi. Beginningless, has an end
  2. Ananta (corresponds to infinity). Has a beginning, endless
  3. Nitya. No beginning, no end (Aristotle's time)
  4. Anitya. Has a beginning, has an end.

Reject what doesn't match with data. A theory's only purpose is to explain data. If a theory cannot explain what happened then it's a useless and harmful theory.

If time had no beginning we wouldn't be here. It's because we cannot get out of an infinite well. We are here so time must have begun at some point.

How to find out that time will end or continue forever? We have to look at what already happened and see whether time is on path to end.

First, what is time? Time is defined by movement. When something changes location only then time passes; otherwise things are frozen, like a movie is paused.

We see the universe moving from concentrated stuff to less concentrated stuff. The universe expands. We see in the past the universe was smaller than it is now.

As the universe expands things get cooled down. Disorderness increase. Potential of doing stuff decrease. We can extrapolate and conclude that if this continues soon the universe would be in a state where nothing can happen. Call it cooling down or increasing entropy. If universe continue this way it sure has a heat death.

  • 2
    The second paragraph is not supported. It just asserts, but the logical necessity between "no beginning" and "we are there" is not there.
    – Frank
    Jan 10 at 15:07
  • 1
    How is second paragraph not supported? If time has no beginning then infinite time must have passed till now. Thats by definition of "time has no beginning". Also, its "we are here", not "there".
    – Atif
    Jan 11 at 3:05
  • "If time has no beginning, we wouldn't be here" is what's not necessary.
    – Frank
    Jan 11 at 15:11
  • @Frank try getting in a room which never start
    – Atif
    Jan 11 at 15:18
  • 1
    @Atif you don't need to get into the room if you're already in it, and the room existed forever with you in it. Does it seem like you are presupposing a beginning in order to prove a beginning?
    – Matthias
    Jan 19 at 19:07

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