You look out of the window and it's raining. You grab a bottle and pour some water into a glass. You probably don't consider these events as necessary: it could have been a sunny day and you were free not to pour water into the glass. If you open a box and find a book inside it, you don't think that your act has affected the content, but just that the box contained a book and you didn't know it.

These two cases, however, are not so different: if it's not possible to change the past (take this a premise), it follows that once an event happened, it could not be otherwise from how it happened.

There is a book in the box, tomorrow it will rain, you'll pour some water in the glass... what's common between these facts is just your ignorance of a certain portion of space-time. It's raining, you pour the water - the fact that it didn't happen otherwise proves that these were the only possibilities (at least in this universe or hyper-time).

What happened is therefore necessary, for the simple fact that it's impossible that an event that took place in one way took place in another. After all, you know for sure that something will happen tomorrow, this is necessary - but you don't know what.

Note: this question is not strictly related to determinism. Causal determinism says that event X is determined completely by previously existing causes. In this case, X can happen even undeterministically. X would be unpredictable, but even before it happens the fact that X happens is necessary, although you may not know what X is. You know for sure that something (X) will happen tomorrow, but you don't know what. For SEP determinism is: "The world is governed by (or is under the sway of) determinism if and only if, given a specified way things are at a time t, the way things go thereafter is fixed [as a matter of natural law]". The brackets are mine, and are the part that I don't need in the case above.

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    en.wikipedia.org/wiki/Determinism Dec 13, 2018 at 11:21
  • @ChrisDegnen thank you, I'm aware of it, the question is related but not exhausted by this idea. Dec 13, 2018 at 11:43
  • @FrancescoD'Isa please explain the differences in your question; or at least what you think distinguish this idea from the generally well-known determinism. Dec 13, 2018 at 14:34
  • @YechiamWeiss Determinism says that event X is determined completely by previously existing causes. In this case, X can happen even undeterministically. X would be unpredictable, but even before it happens the fact that X happens is necessary, although you may not know what X is. You know for sure that something (X) will happen tomorrow, but you don't know what. Dec 13, 2018 at 15:39
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    I am going to remove these comments and fold them up into a long, rambling answer... I hope that does not create too much confusion, but I feel they take up too much space here.
    – user9166
    Dec 14, 2018 at 18:10

4 Answers 4


I suggest a distinction. If X happened - you poured a glass of water on December 13 2018 at 10.57 hrs - the truth that it happened is a necessary truth in the sense that it is impossible for that historical truth to be false now or at any future time.

However, to introduce the distinction, it does not follow that your pouring that glass of water was a necessary event at that time. For all I know we may live in a world of total contingency where anything can and does happen undeterministically. You might even have had free will in a deterministic universe such that you had a choice totally unconditioned by your situation for action and the laws of nature (if there are any).

In such scenarios your action was not necessitated but, once it had been done, it was necessarily true, and in that sense is a necessary truth, that you had done that action.


'Necessity' is not a notion I generally use but I suppress my reservations in the interests of the question.

  • Thank you for your interesting answer! I mostly agree. The event can be undeterministic, but in the sense that we can't predict it. Once it has happened, in what sense it could be otherwise? Obviously it could not, since it was not. I think this is a good way to think it: I know for sure that something will happen tomorrow, but I don't know what Dec 13, 2018 at 11:51
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    @Franceso D'Isa. I entirely agree - it was what I was trying to say. An event may not occur of necessity (and hence be in some sense unpredictable) but once it has occurred it is necessarily true that it has occurred. The statement that it has occurred is in this sense a necessary truth. Best - GLT
    – Geoffrey Thomas
    Dec 13, 2018 at 12:01
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    It might be worth taking into thought the idea that while this may be true to events you remember (subjective, short-time memorized events), you can't necessarily say the same for other past "events" (events that happened in the far past, events that happened to other people). Dec 13, 2018 at 14:38
  • @YechiamWeiss of course we can suppose a skeptic doubt about what's happened in general, but this would open another big chapter. For argument's sake I'd take for granted that the event(s) we are talking about did happen, independently from the witness' reliability Dec 13, 2018 at 15:32
  • @FrancescoD'Isa but if you take for granted the events actually happened, you already pre-suppose half the answer :-) Dec 13, 2018 at 17:07

Suppose that the universe is not deterministic (indeed, this is required for anything to not be necessary given the initial state of the universe). This can take whatever form desired - chance, free will, or whatever other position one can take.

Then an event A which happens at time B is necessary at time C if and only if, in all potential futures of C, the statement "A occurred at time B" is true. When we say that an event is necessary, we are frequently speaking in relation to the beginning of the universe, but other referents are possible.

Directly relevant to your example: you pouring water in the glass is necessary only if there was no possible world in which you did not do so, given (perhaps) the state of the universe that morning.

There is a second definition of necessary, which is related and similar. Something can be said to be necessary if there are no possible worlds in which it does not happen. For example, it is not necessary that there be something rather than nothing, or that entropy exist. It is necessary that logic holds, because it doesn't depend on anything contingent on any particular possible world.


This is a clever idea, but I have never managed to articulate it well. It is encouraging to see someone else express it though.

I think the words "numerically identical" belong somewhere in the explanation. Something along the lines of "A thing that occurs (or will occur) in a certain way must occur in that particular way in all numerically identical cases. If it does not occur or occurs differently then something has changed and the case is not numerically identical". Something like that anyway. Its a bizarre delightfully circular and bizarre but that does not mean its not true. Maybe only circular things can be really true, truth has to start somewhere.


This notion goes back to Aristotle's argument about the sea battle, which is the seed of modal logic. He asserts the Law of the Excluded Middle and applies it to the notion of necessity. From a more modern point of view, this is a modal fallacy. Rules like "should implies can" and "is implies must be" choose a given form of the mode involved. Humans across languages and histories tend to conflate a complex of modes into four: Cardinal, Fixed, Mutable and Actual. But they don't really respect those divisions in detail.

There are plenty of Mutable modes, depending on what you consider 'possible'. You can see them warring in the history of the notion of omnipotence. God can do anything possible, so what is possible? Is the inconceivable possible? the inconsistent? things that are consistent with logic but not chemistry? Are there things that are conceivable to some other form of intelligence, but not ours? Are their better concepts what should determine the possible worlds?

This doesn't totally disappear in secular metaphysics. Modal realists like David Lewis need to deal with how one establishes possibility. If you choose something too strong, you are being childish. But if you accept the Cartesian notion of omnipotence, and absolutely anything is possible, you end up in a world of Menongianism and other horrors.

Necessity, then, in particular comes in a range of flavors (exactly one for every flavor of 'possible'). A couple of those have had currency over long periods, and serve metaphysical purposes. The traditional religious notion of necessity as contrasted with contingency, that goes along with the idea of essence is the most popular after Aristotle's. There are things that could have been otherwise, because they are accidental, and things that could never have been otherwise because they are essential.

A more modern flavor of this same approach is the interpretational framework of "many worlds" or of "evolutionary cosmology". In such a reality it makes little sense to choose a single timeline, "'brane" or instance of the universe as a perspective. What is necessary has to be what prevails across all worlds or timelines. It may be important to segregate the necessary things, like laws of physics, from accidental things like the mass of the universe. Determining what constants are necessary and what are symptomatic of our timeline is an important physical question. If you already accept Aristotle's non-contingent necessity as your model, you have to then create some kind of super-necessity to talk about these questions.

Another version of this is Boltzmann's notion of global entropy distribution, which he proposed to address Loschmidt's paradox, created by the observation of 'CPT symmetry'. He imagined that the we observe time as we do because we are in a low-entropy instance of the universe, where things are osmoticly moving toward higher entropy. In high-entropy universes, time should flow backward as entropy would naturally decrease. But at the same time, our memory is an exothermal process, so even if we were not really in a perfectly low-entropy environment, but a more intermediate one, we would remember only the most recent iteration of our timeline. Our memory would not evolve this way in a different environment, but that does not mean such environments don't exist. (Put cute: By the Anthropic Principle we are the efficient cause of the second law of thermodynamics.)

This gives an interpretation for "what is waving" in the wave-model of particles. In a low-but-not-extremely-low entropy environment, what is waving could be the evenness of the level of entropy. Even though entropy ends up giving us a global arrow of time, it could vacillate a bit first, as local entropy distributions that create local time rates catch up with one another.

In such a framework, the things that happened and then un-happened were temporarily real, so how can we then declare them necessarily otherwise. What is happening right now may not end up in the final timeline. So to declare its consequences necessary is premature.

  • Thank you for your answer and interesting survey through the problem. I've a doubt: even if time can be 'undone', this does not mean that the event didn't happen. It means that it happened and then was undone. In a constant flow (or even 'unflow') of time, nothing can be considered permanent, but even if time could change its course, something that happened can't stop being happened... here's an old question of mine related to this issue Dec 14, 2018 at 20:36

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